It is a long thread and I haven't read everything, but it seems to me that the issues are clear: he has, starting with the school transition when he was 17 1/2, more often than not found himself in inappropriate environments. ASD individuals need, more than anything, consistency and predictability. His needs should be assessed and then met. You know the difference between how he is when his needs are met and when they aren't because you have lived the difference. You can't medicate the issues away, or support them away, when a wholesale change in environment is needed for him to be comfortable.
The problem is, of course, that the above is, at this stage in your lives easier said than done. It breaks my heart to read your words that seem to see what he needs, but be unable to provide it.
I wish I had a useful answer.
But I will say this: stay focused on what you believe he needs, using your gut instinct and experience with him as a guide. Pay attention to what he is telling you, and what he isn't telling you, applying in the analysis all that you know about him. Don't let anyone talk you into a different path, once you and him have figured out which one is best.
Good luck.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).