Try not to freak out too much, Kiki. Have you talked to your son about the dangers of overconsumption of alcohol? Perhaps he already understands to keep it in perspective and will be a normal drinker or a nondrinker.
I had the exact same experience when I was checking out schools, right down to staying in my cousin's dorm with her roommates and having beer. I did develop into a problem drinker and then an alcoholic but much later. That experience had no impact on what I would later become as a drinker.
Perhaps this is not helpful, but remember, he has you to learn from and from whom to receive guidance. Maybe talk to him today, if you haven't already, about the whole thing. Ask how he felt then vs. now. That might give you a better idea of where he is in terms of risk level and allow you counsel him?