I would tell my son - i'd rather you be laughed at than dead.....
But there will NOT be a next time! There is a REASON we have an age for drinking....at 15 he isn't mature enough to know when "enough is enough" - and for that matter - some "grown adults" don't know either!!! IF he feels the need to drink - TALK TO ME!!!! COME TO ME!!! CALL ME!!!!! IT IS OKAY TO STAND UP FOR YOURSELF!! IT IS OKAY TO SAY NO!!!!!
He needs to find new friends - get involved in a church youth group, Tae Kwon Do - or some other group where honor , integrity and respect are valued...these friends obviously don't respect him.
Now, that being said - when I was growing up alcohol was NEVER a no-no for me...my parents gave us the small aperitif glasses of wine, if we wanted to taste something - we weren't told no - by the time I got to HS, alcohol didn't mean anything to me - it didn't "do" anything for me other then give me a headache or bad/nasty taste in my mouth....so the peer pressure didn't affect me - I could go to my own home and have one with my parents if I wanted....
My kids get a "taste" of beer and wine and trust me - they are like YUCK!!!! How can you drink that stuff daddy?! As I don't drink - seriously - I can't remember the last time I had alcohol....
Have an open and frank discussion with him about drinking...if he was at a friend's house - you should KNOW where he was any way - and talk with the parents....did they know alcohol was being consumed?
Tell him it's OKAY to say NO!!!!!! Tell him to tell the people who are pressuring him to drink - I'd rather be alive and laughed at than drunk and stupid....as kids get REALLY stupid when they are drinking....they still think they are INVINCIBLE!!!! They aren't...as seen in MANY teen drinking and texting and driving "horror" movies.
I would also call the principal of the school and ask that they go over the drinking and driving program again - don't get specific with names - just say that with "Prom season" coming up you'd like the kids to have a reminder about the dangers of drinking and driving, etc.......it's OKAY TO SAY NO!!!