My son will complete one year at BYUI at the end of July. He has passed very few of the courses that he has taken. He is probably slightly more functioning than your son, but struggles with many basic things even though he is extremely intelligent. It is hard to watch his struggles, especially since I live 15 hours away and can’t drop in to check on him.
I taught him to make a couple of simple pasta dishes before he left and he had those two meals for breakfast lunch and dinner for a few months.
He had a job as an early morning janitor on campus but slept through his alarm too many times and was fired. He will be coming home at the end of July and is planning to serve as a service missionary.
My wife and I hope that he will be able to function independently as an adult, but we also accept that he might not.