Sorry for your troubles. It sounds like he does have some sort of behavioural dysfunction, whether it's a.d.d or bipolar or whatever. The frustration of this can very easily cause people to stick their head in the sand ie drink or do drugs.
It's just my opinion but I reckon you should try to convince him it's nothing to be ashamed of whatever it is, and therapy or medication is just a treatment for the brain, like insulin is a treatment for diabetes. It can be hard when nearly functioning people are embarrased they don't live up to what they think they should.