I'm in no way qualified to give you a professional answer, but from a human perspective, I'd start by talking to him and expressing the very fears you just described. Make him understand that it concerns you, and that you're there for him should he ever feel that inclination. Just being able to talk about things with other people can be a tremendous help when dealing with stressful situations. Make sure he understands that he's not completely alone.
That's a very good-spirited answer, but not a very realistic one, I am afraid. Most of the people who exhibit problems of that kind ARE talked to; their parents, children, friends etc are, on average, truly caring and do attempt to help. The sad truth is - a lot of people who gave up on 'life' have damn good reasons to have done so. A lot of them would laugh at you if you try to 'talk' to them, providing they have some laugh left in them --- because most of them actually did hear it all already, and it just wasn't enough.
Another sad truth is: substance abuse actually does solve a lot of problems, just not in a "socially acceptable way". But, well - a lot of people don't care about what's "socially acceptable".
