Yes, it is being mean to put him in the kitchen by himself.
He is only 6.
I'd try to talk to him ahead of time before you sit down. Explain that from this point forward we need to see manners at the table. This means we don't talk while eating, chew with our mouths open or bite off too much food.
Set up a sticker calendar. During the meal, gently keep correcting. Compliment him when he is doing something right. Give him his first sticker and say that you are eating with good manners with your mouth closed etc.
Your husband has to be on board with this too. He needs to talk to the ex about this situation.
It is never a bad thing to have good manners in any situation.

Omg he is 6, not 16....yeah I agree that's a bit on the harsh side. He'll eventually get it, you don't have to resort to isolating him in order to teach him the right way...that is so old school. Just keep trying, unless he has some issues you are not aware of, he'd eventually come around.

I think there are bigger issues to worry about than this in life....quite frankly, I'd worry more about your son being so upset by this that he ran to his room crying. My daughter (16) has a condition called Mysophonia where certain noises, particularly chewing and breathing noises, cause her to react in a way that is so off the charts it is scary, she now goes into an instant rage (it's also referred to as "Sound Rage"). Not making a medical diagnosis or anything here, but it strikes me as a bit odd that a 10 or 12 year old boy (you didn't specify which age your son is) would be so bothered by this that he ran crying from the table. FYI - mysophonia typically does not start to rear itself until late late adoolescant/early teen years....

Mkroopy - thank you for your knowledge and experience, I typically agree with your advice to others. Yes, he is 6. That means he has been eating for 6 years, 3 meals a day, that is 6570 meals and a lot of chewing. He has had plenty of time to figure this out. Expecting good table manners and enforcing them is not old school. Last time I was tempted to take his plate away and throw his food in garage. This is unexpectable, it is disgusting and gross. When I look up and see it, I want to vomit myself. I don't blame my 10 year old for getting upset, this has been going on for a year now and frankly, I think he does it on purpose sometimes just to be brat. He told his mother and she told him he can chew with his mouth open if he wants!!! She has always wanted "dad" back and now has boyfriends in and out, on and off and doesn't like that her son's dad is with me and that we have more than she does.

Besides all that...yes, a small thing in life to worry about in the grand scheme... this in part it has gone on for so long. He disrupts our home in all ways, his behavior is poor all around and he is very disrespectful to myself and my kids. Harmony is important and we don't have it when he is in the home. Trying to tackle one issue at a time and I feel like I am doing it solo; when in fact, I am not even his step-mom and his Dad is not doing it himself on a consistent basis. I talk to him about it often, he does well for a week or so, then stops until I bring it up again.

Blended families are so hard, the lines are so thin. I am frustrated and sad.

"Expecting good table manners and enforcing them is not old school."

No, but the method you are describing (isolation away from everyone else) is a bit old school I think...look I get that you are not suggesting beating him or anything...I just think that at that young age, isolation is a little severe. He might not he really able to control that part of his behavior effectively yet, all kids progress differently. Please don't make the mistake of comparing him to your kids at that age...you may be setting him up for constant failure, which can be damaging to his development in the long run.

" Harmony is important and we don't have it when he is in the home..."

It's very important to me too, and I don't have it either when my daughter is at my house (split custody w her mom). Like I said, she is 16 and her mom and I have spent untold tens of thousands on therapies (inpatient, outpatient, group, etc...), medications, alternative schools and as of yet nothing has gotten better.

All I am trying to say is that all kids are different, while YOUR two kids may me more likely to learn expected behaviors quickly, this is not the case with all kids, and it is NOT 100% related to parenting styles (of course it can be a factor). I cannot tell you how many times over the years I have been discussing issues with my daughter with an acquaintance and had them say "oh if my kid did that I would put a stop to it quickly..." My god how that burns me....kids are all different, even ones without anxiety and personality disorders like my daughter...parenting is not the case where what works with one child will necessarily work with others, I've been biting my tongue on this issue with family and friends for years. My brother has two daughters (older than my daughter) that were those low-maintenance, high-achieving kids. Always pleasant, never in a lick of trouble, high honors, great kids...one is an engineer now and one is in pre-med. I am very happy for him, but it makes me want to take a shovel to his head when we discuss my daughter and he starts to offer advice like "well what always worked with my kids was......". Like he knows anything about dealing with any child crisis.

Just keep in mind he may not be like your kids in terms of development....this might even out in the long run, and might not...you have no way of knowing at this point.

Point well taken Mkroopy! My heart goes out to you and your daughter for the struggles you are facing!

Thank you Dawn, it's been a tough few years. She is beautiful and healthy and intelligent, and deep down a good kid (i.e. not a "troublemaker"), but she is just really struggling with herself and her anxieties. Hopefully all the help her mom and I are trying to give her will eventually pay off....

No wonder the poor kid has some issues with all of the grownups drama going on! Yes, he has had 6 yrs. of eating and meals but what difference does that make if no one has taken the time to show him proper manners? I have seen 18 yr. old kids with bad eating habits and it all goes back to the parents not doing their job! Clearly he has a lot of stress in his life. Put yourself in his shoes and think how you would like to be treated and act accordingly.