I would say he's just being a teenager and trying to fit in, but I was a 15 year old alcoholic, so it's not always just 'being a teen'. I'd suggest talking to a therapist about it, or a drug councelor. You can also go to google.com and search for 'teen drinking', 'teen drug abuse', etc, etc... You'll find A LOT of stuff by doing that.

I wish you the best and hope he's just going through a phase in life right now. Best of wishes and prayers are with you.