when it is a good time to place them in a group home. 
 

No job, needs more help than most average adults can provide, and 18

Not sure how you're going to do this with your SO not in agreement

Is your SO still sleeping with his son? Because until that stops, I don't see your SO as ready for his son to move out, let alone they young man himself.

I care for her almost solely.  Although we have never and never intend to put her in a home, there is a program that is available in every state called IHSS which is intended to keep people out of institutions.  It's more cost effective for the state as well.  I happen to live in a state where they allow the parents to be the caregiver but not every state allows this.  Either way, they will pay for caregivers to care for him.  I would highly suggest looking into it if you haven't already.  My sd is deaf but she is very loud so I understand what you mean but I do a shhh signal with my finger and she knows to quiet down a bit.  I'm not sure how helpful this is, but feel free to message me if you have any questions.

SS is 21. What does he do all day? Is he still in school? Is he participating in some type of work program? What does DH think will happen to SS if something happens to him (and you)?

I have a daughter who has Down Syndrome. She moved into a group home about 6 months ago when she was 20 (she's now 21) and she absolutely loves it! Why does youir SO persist in wanting to keep him joined at the hip? What kind of a life would that be for his son? Stuck with parents all the time? No independence, no friends, no activities... What a sad existence that would be.

My daughter is thriving in her new home - she has friends, there are lots of things for her to do, plenty of activities are organized and she is getting a slight taste of the working world. We still her for holidays and once a week when we take her to her speech therapy. I am very happy for her and feel that we have all been very fortunate to have found this place.

My advice is to do a bit of research, find out what is available in your area and then present it to your SO as a list of positive things these places can do for your SS and a list of reasons why him being stuck at home with you is absolutely not good for his development. 

If all else fails, ask him what will happen to his son when you two are no longer there and don't accept the answer "oh, somebody in the family will surely take him in" because that is not fair on the rest of the family nor is it necessarily true. FWIW, my daughter also has a twin sister, a standard model, and the last thing I want is for her to feel that she is her sister's keeper. She has her own life to live.  Her sister is our responsibility and always will be.

You DID NOT know what you were getting into. All of us feel it's going to be normal. But as see here it's never is. You did your part.. he's21. Things are not going to change.  You have to change.  You have to put your foot down and say either SS goes or they both go.   I would not have my SO sleep with his kid instead of me.  You must realize there something not right with DH. 
'Once again nothing magically is going to happen here. It sucks for him.. But you are at crossroad.  Either you spend the rest of your life taking care of this kid.  Or you put him in a group home and live your life.  As in going on Adul7 vacations, cruise,  Have friends with out SS involved,  I bed you don't get invited out to dinner with friends because of SS. 

'The faster SS is out the better it is as in "This is the first day, in the rest of your life"

21, even if special needs, is entirely inappropriate to cosleep.

Coddling, even if special needs, is never in the best interest of an adult aged child.  Or any aged child for that matter.  This young man needs a group home where he can have a life and engage with others in similar life situations.

In my first career when I ran my own company I employed multiple special needs adults group home residents in back of the house roles. They were reliable, engaging, and an important part of the team.  A few even progressed to front of the house positions where they interfaced with my customers.  More than one of them would check in when they came to work and tell me that they were going to kill me.  Not a menacing comment, just on par with their mental capabilities. Not one of them ever said that to any other of my team or to my customers.  I was the one that was greated with the "kill  you" statement.

Some of my special needs employees were middle aged and their parents were deceased. They did have siblings that were part of their  support but they did not live with those sibs. The group home was a wonderful place for these adults to be adults within their ability to function as adults, they had friendships, their own efficiency "apartments", and structured activities with professional care giver oversight.  

Many parents of special needs adults forego that opportunity for their adult children, sacrifice their own peace of mind and lifes for their special needs adults, and interfere in the lives that their special needs adult children could have.

Of course not every special needs adult is capable of functioning at that level, though IMHO even those people should have the opportunity to engage in an appropriate residential community and have as quality of a life as possible.

Your SO and BM are not doing their son any favors IMHO. 

Your SO is failing you by expecting you go join him in ruining his kids potential and denying his son, himeslf, and you your best lives. 

If he can't pull his head out, you need to move on.

IMHO.

Take care of you.

Man who is still sleeping with his son.  Like WHY ..  How screw up is DH .  How do you allow this insanity to go on ?? 
'I admitt I am screwed up also. But never that screw up.  You don't go on adult vacations ? By yourself.?

I would maybe start by bringing up the fact that parents rarely outlive their kids. That the time will come when you both are gone and SS might be at the mercy of people who don't care about him. And that the sooner SS finds a place where he is happy and well cared for, the better. He will adjust better the younger he is. Also, you and your DH are still well enough to look after him and make sure it's a good fit. If it happens when you guys are gone, SS will be at the mercy of who knows who. DH seems like a "rescuer" personality and likely codependent if he still sleeps with an adult man. DH needs to feel like, by taking action now, he is, in fact, "rescuing" SS from an uncertain future. 

Though we did not know it when he was a minor, SS-31 is ADHD.  We raised him with very rigourous standards of behavior and performance.  After graduating from HS at 17 and with honors, though it was a periodically arduous battle, we worked his tale bone off as our live in chore boy until he figured out that working his butt off for nothign more than a roof over his head and food was not what he wanted and enlisted in the USAF.

He is approaching his 13th service anniversary (I have been saying 14th anniversary but... I took a year of credit not yet earned).

He is fully capable of supporting himself so we do not have anywhere near the challenges that you and many others have.

One of my childhood BFFs was Dx'd with schizophrenia in his mid to late 20s.  His parents did have him institutionalized a few times over the years and have had him made a ward of the State.   He lives in a garage apartment in his parent's home and has for decades.  

His parents are elderly and have their estate set up to go into trust for his support upon their demise.  It is administered by a bank that their family founded.  His younger brother is not capable of protecting his best interests and would gut the trust if made the executor.

I have another very good friend whose special needs adult son could not live on his own.  He remained with the family.

When I was running my own company I employed a number of residents of various group homes in back of the house jobs.  They were great. Quirky, reliable, etc....   They lived wonderful lives in their private lives in their group home.  Close friends, community, a level of independence, great care, etc.....

I hope that your DH and BM can get their heads engaged and do what is best for their kid.  Though the cosleeping with his adult son daddy is not likely to engage his head effectively.

Good luck and take care of you.

I've been working in the developmental disabilities field for 25 years and my advice is this.

I've seen FAR too many families keep their disabled adult children at home for as long as possible. While I understand, what ends up happening more often than not, is there comes a time when the parents can no longer care for the adult child, other family members are in no position to take him/her in, and the diasbled adult ends up in state custody after the parents experience severe helth problems of their own or die. Typically, that means they're placed in a bed in a hospital or nursing home for months on end until a bed in a group residence becomes available. Even then, it may not be the most appropriate setting for that individual, it may be far from other family, etc. Meanwhile, the adult child is not only grieving the loss of his/her parents, but also having to adjust to a new home, usually enduring several temporary settings in the meantime.

I implore parents and other family members to get their children into appropraite residential settings early on, whether that's a group home, supported living situation, whatever they need to be happy and successful. That way:

a) the parents have say as to where their child lives and 

b) the child can get used to the new home, let it become home, bond with peers and staff, while still being a part of their parents' lives. When the parents inevitably, eventually pass, it will make the transition SO much easier.

Plus, a lot of adults with disabilities thrive in an approipriate residential setting. They make friends and enjoy social activities with peers who are on their level. They go out in the commuity and learn skills they need. It's great that he's attending a day program four days a week; that's an excellent start.

I've worked with many, many people who thrived and developed great self-confidence after leaving home for their own living situation. Even if it's a group home, they consider it "their home" (because it is) and makes them feel more independent. Please, please consider the future! It can take any number of years on a waitlist in some states/areas, so keep that in mind.