I'm wondering if he can live somewhere that can care for him? I know some nursing homes do but he is still young. There must be places for disabled younger adults too. Does he has a case manager and get Social Security? If not, he would qualify.
You can't take care of him his whole life and he will need a place that can help him. He may like the idea of moving out.
I am not at all sure that a psychiatrist can help somebody with autism since they don't communicate well.
I wish you luck and send prayers.
