Why does his mom encourage or allow this?   Does he have developmental issues? substance abuse issues?  

Does he have to pay rent? 

I would start there. What we do for my special needs SS (22) is he has to pay rent but we put it in a secret savings account. One day if he is able to move on his own, we will gift it back to him. 

Maybe if you wife would be willing to do that. But also in paying rent he might be more motiviated to work more and/or move out. 

I have a similar situation as you. My SD18 will be 19 this winter. She still lives at home with BM. DH never made her do any chores, spoiled her, etc. I fully expect when BM kicks her out, which will be during their next big fight, she will call DH to ask if she can live with us ( or just show up). 

I have made it clear to DH that SD will never live with us as an adult. She is 18, so she can damned well get a job. If he wants to get a second job and use that money to pay her apartment rent, deposit, and food - then that is on him. If she tries to move in I will change the locks and both of them will be out on their ass and our marriage will be over. That is the only way I could get him to understand the seriousness of his actions. 

You may want to start laying the groundwork for this plan yourself. Sounds like otherwise, you will never be rid of this kid.

Your wife is your big problem here. She has to stop treating him like a child and treat him like a young adult.

There is a transition period from 18 to 22-25. The "old days" of kicking out an 18 year old and telling them to fly isn't very realistic anymore. Housing is expensive - provided you can even find any. School is expensive (at least in the US). Wages are...eh. Utilities and food are expensive, even for middle class professionals. So expecting an 18 year old who just finished high school to immediately jump into the real world is unrealistic.

HOWEVER, pushing them towards independence is definitely a need at this stage of life. Full time employment or part-time employment plus schooling is a NEED. Having them pay rent, even a small amount, is a NEED. Making them take care of their own dishes and housework is a NEED. And the consequence for not doing these things is getting a 30 day eviction notice.

But your wife has to parent her young adult son and push him to do these things. If she won't, he's not going to because 1) few people want to grind all day just to survive, and 2) his parent hasn't taught him how to do it. Few people overcome how they were nurtured, so you can't expect him to behave better than he has been taught.

Yes, this might end your marriage. Does your wife realize this? Like, REALLY understand that is where this is going? If not, time to make that abundantly clear. If you have joint finances, time to separate them and make her pay a greater portion of the household bills to accomdate her son. Go to therapy on your own to help you navigate the emotions through this. Consult an attorney to figure out where you stand legally. Keep telling her calmly that this a problem you want to solve but you won't live like this forever. Give hear a list of expectations and a timeline. If she can't work with her son to meet those expectations (and they should be reasonable - giving him 3 months to find full-time employment, 6 months to get his name and deposit down on housing, a year to be moved out), then you walk. And walking at that point should be easier because you'll have already divester your money, talked to an attorney, and set up your exit plan.

Really, the decision on whether this impacts your marriage is on her. What you have control over is whether you want to stay or not, and how much compromising you're willing to do. 

What happens if she died tomorrow? Who is going to take care of her boys? They are unlikely to want to stay with you. A mother's job is to set her children up for success. To be able to cook a couple meals from scratch, pay their bills and understand long term consequences for their actions. 

We are all one breath away from the end of our lives. 

You have a wife problem, not an SS problem. If I could work 20 hours have have everything done for me & I could do what I wanted, I'd sign up in a heartbeat. Who wouldn't? 

launch at 18yo.  We offered him the mom and dad full meal deal college scholarship ride anywhere in the world he wanted to go, that he could get accepted to.  He did not apply to any colleges. He did not have to apply to the Military School & College he attended for HS.  He could have gone there for JC then transferred to a 4yr school after completing his Associates.  Unfortunately the Spermidiot helped SS hack the school firewall and they stayed up all night every night playing WoW so SS was comatose in class every day and failed the classes he needed to graduate.

So we withdrew him from the Mil School mid year of his Sr. year, ran him through our local HS where he did graduate on time and with honors.

At that point it was burning platform time. We lit the platform and kept piling on the fuel until he launched.  A chore list from hell that kept growing nearly daily.  He was our live in beck and call boy/chore bitch.  He cooked, cleaned, washed, filded, etc, etc, etc, etc, etc.....  If he did the daily chore list he got to do the next days.  If he chose not to finish it all, he was on the curb when we left for work and didn't get back in the house until we got home after work. Then he had to do the stuff he didn't do the day before and what he didn;t do while sitting on the curb all day.

After 4mos he enlisted in the USAF on the delayed entry program. He still was our beck and call boy for 4more months until he reported for BMT.  He has been in for 11.5 years, has 4.5 yeares left on his current commitment, and plans to re-enlist to get to 20+ and retire when he is in his late 30s.

He is doing very well, has completed his AS in Computer Science, and is working on completing his BSCS.

Parents who enable slug status in their unmotivated young Kidults, are not doing their kids any favors.

SS is very appreciative that his mom and I forced him to launch. He is a man we are very proud of.