Name change as I'm going to sound awful here.
There can be an alternative @thegreenlight. Not for everyone, but for some.
We have four DCs, all of which are ND to some degree but two severely.
My eldest DS is now 18. He has autism and ADHD and has been violent and unmanageable all his life. Like OP describes, he has never has a friend and has not been invited to a single birthday party in all those years. He is capable of mainstream school but was constantly in trouble. He was excluded from schools three times (or would have been, but each time the HM called us in for the "we're not sure this is the right school" conversation so we jumped before we were pushed). At home he was aggressive towards DH and me and frequently physically assaulted his three younger siblings. DH intervened when it got physical (only to prevent him getting to them) but as DS approached adulthood it became clear that DH (in his 50s) may not always have been able to protect them. For the last two or three years we didn't cope in any meaningful way and in the end we allowed him to spend all his time he wasn't at school in his room because he was sort of happy and that was easier. Nobody cared. Since he was five we had door after door slammed shut, no support whatsoever.
When he was 17 and a half his college suggested we try adult services at our local council and they have been amazing. They couldn't do anything formal until he turned 18 but they could help us prepare and days after his 18th birthday DS moved into supported housing about 45 minutes away from us, initially for short week placement in a special unit to help young people transition and then into a "permanent" home with four other young men and full time support. This is all funded via UC and housing support.
We speak to him every day and typically see him about twice a week for a couple of hours, either to take him out for lunch or because he has an appointment or, to be honest, because he is in trouble. He shouts and swears at us as he always did, but somehow it's manageable as we return to a calmer house and he goes to his. The carers who live in the house in shifts are saints.
I know this will sound like we are terrible people and have failed him as parents, but to be honest I think this is the solution that has given us an option to continue in a relationship with him. Otherwise I just don't know how we would have coped with things on the downward trajectory they were on.
I guess I am posting this because OP's DH said they will eventually move out and lots of people have doubted that. It may not be possible, or appropriate, or desirable for many, but it can happen. OP's description of life with her DS struck a lot of chords with our own experience so I thought I would share a tiny bit of our own trainwreck journey.
Bracing myself to be torn to shreds.