Well at least he is old enough to have a phone and use it, that right there is so important.
I'm guessing he knows about drinking and that he needs to not be around people who are drinking? If not, that is going to have to be worked in to a conversation. He has to know so he can call you to come and pick him up if need be.
It sounds like you have done as much as you can at this point don't you think? Until he steps out of line, what more can you do.