L.B.
My husband's family would allow my husband to drink alcohol on certain holidays or family reunions but it was always to remain "in check" and no drunkenness. So when my husband was approached my classmates to come to a "great New Year's Eve party" that was going to have beer he just said, "no thanks I've got better plans and better drinks at home."
I guess the point that I'm making is kids are going to try things one way or another...you can either be open to his experiences and be present so you can monitor them or you can be a staunch advocate of them never doing it and have him go behind your back anyway. Frankly, I vote for the openness and honesty route. Now I by no mean advocate underage drinking but if he's going to try it or wants to try it, isn't it better to do it with you in a safe environment than with his friends who may also be drinking AND driving?
Besides, you don't know if this is a problem or not. It could just be teenage rebellion and a try it out phase. I would sit down and have an honest conversation with him. Try not to let the emotions run rampent...you may not be happy about it but if you get fly off the handle angry he's going to bottle up everything and not feel like you are approachable or that he can even have a fair conversation with you which is where I would suggest starting out. I would try a Love & Logic approach on this and let him set his punishment, if it's seen that he deserves one.