ADHD can make people mature later than others. With this in mind, he is 22, but with
ADHD, maybe he has the maturity of a 15 year old. I would imagine that's kind of tough-- being expected to act like an adult when you have the maturity of a teenager.
However, maturity level aside, he is 22 years old. He is legally able to make his own decisions-- and unless you think that he is a real physical danger to himself or anyone else, you can't do much.