L.A.
Good job mom!
He did what you have asked. He called when he did not feel comfortable..
I know here in Texas an underaged drinker is not allowed to be around anyone else that is an underage person drinking at the same event. Even if a parent provided it.
There are so many possibilities and he made the right choice.
I would do as the other moms said and speak with him about how good it makes you feel that he knew what needed to be done and was strong enough to do it. Let him know this is probably going to happen even when he is of legal age and something is going on he knows he does not feel right about, and he will have every right to get him self out of there. This can happen at school, work, social situations or out in public.
And if he has anything he ever wants to ever speak with you about it will be fine.
But do not push. He did exactly what you asked him to do.. You have a good kid.