I am not a step parent so I hope it's ok to answer. I have stepparents though and so I know a little about the relationships.

I honestly hate to steer you towards 'leaving' your husband so I will suggest this. Marriage and family counseling? I am a huge advocate of therapy. I never would have been before but in the last 6 years of my life I have had and still have many types of therapists. I saw a therapist for myself during my seperation. ExDH and I saw a therapist for coparenting. I saw a therapist during a very rough time in my life a few yrs ago and I am seeing one again Now. Therapists offer different points of view to help to understand where someone is in their thinking. It's especially beneficial when you go together because it gives you a 'safe zone' to talk about the things that would normally cause a blow up or someone to just walk out without the other person being heard. If your husband is completely against that, what choice do you have? You have two other children you have to consider that are CHILDREN. Your adult child and this 17 yr old are old enough to handle themselves but the younger ones need to be considered.
If your SS is disrespectful in your home, at 18 it will be even worse. He probably needs more counseling than anyone. Sounds like he is on a distructive path and needs some immediate guidance.
You aren't just an SM, you raised him. You deserve the same respect as any parent regardless of your blood relation. My stepfather has been my DAD for 27 of my 30 yrs. My biodad watched from the third row while my SD walked me down the aisle and raised me. That was how I showed him my appreciation while my 'sperm donor' ran around taking care of his 'other' family and didn't blink in my direction. I know that the teenage yrs are rough but your husband and SS should be showing you a little more appreciation for the unconditional love you have showed over the last 13 1/2 yrs.
Your husband should definitly be showing his unconditional love and supper for your DD as well.
I had a similiar conversation with my own stepdad today. It was the best choice for me to divorce my ex and have my dd in two different homes instead of in the one home where she would never learn love, affection, acceptance, and support. Which is what she sees now that I am remarried and absolutely in love with my husband. I know you probably aren't thinking about that aspect of it but is it healthy for your DD and DS to see this and more importantly feel the back burner, day in and day out?

It's up to you but I hope that will help you

good luck

Your ss's behavior could be partly from being teenager who is trying to establish their own identity and entering into a phase of being independent.
Having said that, I still do believe there are BOUNDARIES for family that CANNOT be crossed. For example, playing stereo all nights and disturbing the family's sleep is something that cannot be crossed. The solution is (1) to have him use headphone ($20 from best buy) or (2) stop playing music at mid-night.

Being argumentative is perhaps just a way to being a teenager (all my children have done that). As long as they don't get physical then just let the argument die from lack of oxygen (don't feed the fire w more fuel).

Be clear but not vindictive about his moving out at 18. Again set conditions, otherwise if he becomes homeless or thief, or drug user then your H will forever feel that GUILT. Make sure your H is on the same page w you, (1) sit down and set the time line and expectation, (2) he needs to begin to find jobs and save so much per month while he still lives at home, (3) he needs to help out w rents and foods from 18 and on, while he saves up money for moving out in 6 or 12 months.
I know, being parent is a thankless job and often filled with FEAR, OBLIGATION and GUILT. Since I shift my responsibility from making them happy in life, from just being a sheperd who teaches them how to fly and where they fly and how they fly are up to them, I have become much more at peace with myself.

Thank you all for your feedback, I feel that my SS behavior goes beyond a normal teenagers rebellion. There are quite a few issues I left out of my original post, but I will name some of them now. He never leaves this house. He stays in his room, playing video games. When I do get a break from him, (he goes to his moms house on the weekend, WHEN she feels like it) he rings my phone off the hook all weekend long - just to say "hi" to "daddy". I think my H and SS relationship is strange, and I don't see my SS ever leaving and going out on his own because he has been babied, and spoiled my my H, and his family. That's fine, as for me, the idea of him staying here past 18 makes me physically ill. This morning,for instance, he gets up for school, pees allover my toilet seat, and floor, lets the dogs out, leaves the garage, and back door wide open, and leaves the front door wide open for any idiot to come on in to my house. I won't say anything to the H because he won't do anything about it. My SS also got into the cupcakes, and candy and made a mess in the kitchen..he's 17 and knows better, I think he does it because he knows I don't like him, and he knows it aggravates me, and he also knows he can get away with it. As for him becoming homeless or a thief, my H doesn't care if my 19 year old is going through these things, so why should I care about his? - not really my problem, the way I figure it is, the H should have been preparing his own child for the world, instead of spoiling him...I am literally done with the entire situation! I'm tired of my kids feeling as though he is above them, he isn't, far from it. But I thank you all again for your feed back, I will consider all of your advise, thanks again!

How does the SS function at school. With other classmates. What does the school guidence office/counselors think of what's happening/not happening with SS outside of the home setting?

Just curious as to if DH has ever had an eval on the son. While some of the stuff could maybe be thought of as obxious teen, I'm not sure all of it can be. Could SS be mentally challenged/ an intellectual disability? Nose picking and wiped on chair would not be obxious tyipcal teen behavior for starters. Hiding in one's room and calling 'Daddy' all day would not be either.

Yeah, I know that does not 'help' you or change what is going on for you, nor you being the one to deal with his behavior, but it might account for why a almost young adult seems trapped at an immature age 11. A few things you've mentioned just does not click with 'teen' behavior.

IDK, just tossing it out as something for your husband to consider.

I agree with you, JMT. Some of this boy's behaviour goes beyond normal teenage obnoxiousness. From Brook's description, I would presume that he was developmentally delayed in some way.

My SS is 16 and sometimes displays very immature behaviours -- like an 11 or 12 year old. He was diagnosed with ADD when he was 7 and he's still taking medication for that (very low dose though, just 37.5 mg per day), and perhaps that is the source of some of his reversions to childlike behaviours. Has your SS been evaluated?

He was on medication, ritalin, and aderal, but totally flipped out with it..he could not sleep..I truly feel he's never been made to mature, or to face responsibility, or accountability. He certainly does not act 17 though, for whatever reason, but H won't deal with it, neither will mommy, they will look to me, and frankly, I've done enough. I have an emotionally challenged son, and a daughter with chronic physical illness and I have put them off for years to tend to this kid, and I can't do that anymore. Believe me, washing my hands at this particular time of this kid is my best option, I'm sure of that now more than ever..I now realize I have stepped into someone else's mess, and made life easy for all of them, except me and my children, who will never measure up to this kid, and it's not fair to us. I know nothing will change, if I even bring this stuff up to H, it starts a fight, he gets so defensive, but like I said before, will slice my son to ribbons, and criticize everything he does...in case you haven't picked up on it yet, I am completely fed up, p*ssed off, and frustrated, and am compelled to take action for the sake of my own sanity, and my childrens mental, and emotional well being. Thanks for the advice you all, keep it coming, your words are making me stronger emotionally, and mentally and I really appreciate it!

His behavior sounds abnormal to me, i work with teenagers, flunking classes is not abnormal per se, but whiping buggers on your furniture is not normal behavior at all. Your DH needs to take him to a psychiatrist. Aldo if he is precsribed meds, he has to take them or adjust dossage, taking kids off meds for no reason is a bad idea. can't sleep, then he needs different meds.

Kicking him out at 18 is nto going to work, where is he going to go?

Now, saying 'daddy" could be normal. Sd is 29 yet she calls SO daddy (especially when she wants money LOL)

"flunking classes is not abnormal per se, but whiping buggers on your furniture is not normal behavior at all"

OMG!!! PO1, you raised a daughter... lmao

I've seen grown men do worse. I wouldn't say it's "normal" but it's not any more abnormal than flunking classes. Some males just enjoy being Gross! (hopefully they outgrow it before marriage)

Thanks for you input, he has a mother he can live with her..she has had it easy for 13 + years, seeing him only when she feels like it. She is more involved now, and I think his place is with her.Not to sound mean, but I don't see my H worrying about where my kids would be going, all he's concerned with is the fact that my son is not at our house, even before he was 18, my H never wanted him around, because of his emotional issues, so if it was good enough for my son, it's good enough for his. As for the meds, H decided to take him off of them, so I respected his decision, and He would only misbehave at school, around daddy, or mommy, the three areas where he could get away with it. Funny, I told him to do something, he would do it, no questions asked. So I don't think ADHD, ADD, etc is the problem, unless there is a pill to cure spoiled brat syndrome LOL. I know picking your nose and wiping it on stuff is not necessarily normal, or abnormal, but I'm not really concerned about that, I'm more concerned with the disrespect of my stuff, and the idea that he feels entitled to to it. And I will admit, everything he does bothers me. He can just walk into a room, and I want to choke him...I have a lot of resentment, for SS H and SS mom I am pretty much done with all three of them, thanks for all of your inputs! I really appreciate it!

I feel like this woman has been living in my house for the past 10 years. Please tell me things got better for you?