I understand your frustration and yet, I'm confused at the same time. My situation is similar, my SD isn't nearly as "disturbed" as you say your SS is and she can be very sweet when she wants, so I understand all that. But, it's hardly fair to say he was "thrust into your lap" because you say you married his dad so he could get custody. That was YOUR choice and I've said it before, the adults need to think hard before they get into these situations, especially when you don't have children of your own. It's hard to know how to be a step parent, even when you have your own kids, but not having kids makes it harder to grasp what it takes.

My guess it that you want help or advice but I don't see the question in your message, I just hear a lot of frustration and venting, which I completely understand and hope you don't get attacked for it.

You should take a breather, and take some time to think of a strategy to deal with some of the problems. If you can talk your husband into taking a parenting class, it can really help. Kids need structure and limits and most of all, consistency. Your husband has to be involved because, while you may be given the task of taking care of him while he's at work, it can be overwhelming. If he's not in counseling, he probably should be. and maybe put him an after school daycare one or two days a week.

It sounds like your heart is in the right place and you love your SS, but your frustration has reached a breaking point... So maybe it's time that everyone involved needs to have someone to talk to about the situation, preferably someone trained as a therapist. That way, you WON'T have to sacrifice your sanity to help your SS.

If you can't afford or don't want to talk to a therapist, talking to a close friend, or your sister, etc, might help. Preferably someone who also deals with a challenging child and can help put things in perspective and offer advice?
Check what resources your SS's school offers to him in the resource room to help him with his problems. Maybe your state or province offers some subsidized child therapy program for kids with ADHD?

You mentioned that you ask your husband to help you out more... But maybe he didn't understand the breaking point you've reached - he should be placing high value on your sanity!! I'd probably refrain from telling him that you want to scream at SS - IME sometimes men don't understand the way that frustration builds up in women - he'll might take that the wrong way... but he needs to know that you're at the end of your rope right now and you NEED to work out a plan to take some weight off you. Maybe that won't be much to do with your SS, but just keeping HIMSELF organized and dealing with his own legal notetaking? Or maybe you don't mind that job, so you need him to help SS with his homework and evening routine so that you can focus on the legal stuff? Someone else on here recently suggested making a list of tasks and who does them to see how the labour is divided. I'd agree with that, and maybe add on another column for how that task makes you feel. You might not choose to show you husband the lists you've made, but it might help you sort out your own thoughts to direct you to where your focus-on-sanity needs to go.
If you need to, take a weekend away to visit a friend in the next city without your husband or SS to recharge your batteries and clear out your cobwebs.

I have a tiny inkling of how you feel about the extra challenges that ADHD adds, but I only see my BF's 8 yo about 2 days a week, and his ADHD doesn't sound nearly as bad as your SS's. Here's what I do know:
A__ has benefited enormously from visiting the resource room at his school twice a week to work on ways to manage his emotions and stay focused on a task or conversation.
A__ also has that wanting to please to the point of being invasive, but having no idea how. I don't know if that's part of being a 8 or 9 year old boy, part of being shuffled between caregivers, or part of ADHD, but it's really tough on new caregivers! We try to make the things that will please us very clear: "I would be thrilled if you finished your homework before dinner" rather than "Get your homework done before dinner."
A__ doesn't seem to understand implied parameters, so we have to be clear on that too: "You can only watch one movie this evening" will likely be obeyed without too much fuss, but "I don't want you watching too many movies tonight" is subjective and he'll FIGHT to get to watch two.

Hopefully some of those ideas might help...
Good luck!

This doesnt seem at all like a disapline issue to me.If this child is emotionally disturbed,then he NEEDS professional help,and will not change/act better until that is happening.
Has he had ANY therapy or is he on ANY medications?

Maybe he is just one of those kids who was born like this,but more then likely something has happend to him that has made him like this.Whether this is his reaction of his parents not being together to get attention,or maybe he was abused by someone.He needs to be in therapy to find out what is going on with him...

I know how you feel because I have been around kids like this~the kind that are VERY IN-YOUR-FACE and loud.I'm a quiet,reserved person so it tried my patience to be around that type of overly demanding child.
While it would certainly be rude and uncalled for to just say,"SHUT UP",there is nothing wrong with letting the child know they are bothering you.Something like,
"Lower your voice we are in the car"

Or,"It is not nice to get right up in people's faces so please back up some,ok?"

I agree with imamommy too though that you knew what you were getting into before you married this guy.
I have a question though.What is so wrong with this boy's life at home that you and his dad ARE actively pursuing custody of him like this?
Unless where he is at is really bad(which you didnt really mention)Why would it benefit him to live with you guys knowing you feel this way about him?

Quick:
1. family counseling
2. individual counseling for the SS (if not already doing it)
3. Parenting classes for your husband AND you

You are young to be dealing with this situation and although you think you know what you're getting yourself into, do we ever really?

It may feel like he's ruining your life, just realize that you don't want to be the cause of ruining his life (sounds like he has plenty to deal with already)

BTW, people with ADHD often have social skill problems, it is part of the disorder. Be patient and educate yourself so you can help him, but more importantly help YOURSELF... remember 'knowledge is power' use it wisely.

Good luck