If you can listen to his story without flying off the handle, you are laying the foundation for the important steps that follow. Then, simply and clearly, tell him your rules about underage drinking. Many junior high and high schools help to organize parent networks like this. It can simplify life for teenagers if they know they're all being held to a common standard and that their community is made up of adults who care about their health and safety. And let your son and his friends know that they can call you to pick them up any time day or night if they ever encounter an unsafe situation.
Teenagers often can hear a message from their peers that they refuse to hear from their parents.