You will not be able to fix him. Mentioning that you're worried that he's getting into dangerous territory is a good way to bring your concern to his attention. Ultimately, it is he that will have to make a decision to change. You can simply be a good friend by making yourself available to him.

I started going to Cocaine Anonymous and AA meetings with my Dad (he is the addict, I was being supportive) when I was 17 years old, and I've got quite a bit of insight into this sort of mess. Spent may years thinking I could help, but ultimately I learned (and was told countless times by addicts other than my father) that it's not a problem that can be solved by anyone but the addict. You will not save or fix him.

May want to check out...

http://www.racingforrecovery.com/

~Matt