
Quoting
LawResearcherMissy
Yes.
Beating someone up is still illegal, even if the victim was falling down drunk, stoned out of his gourd, and well underage. Your son being underage and drinking does not void the crime of assault.
Pardon my forthrightness, but not calling the police and pressing charges because your son might have been drinking is dumb. If he has injuries serious enough that he requires medical care - and you really need to have him checked out! - the police need to be called, period.
It seems to me that it would be a little late for him to be cited for underage drinking, as no police arrived at the party, but that's a question for the attorney you really need to be consulting about this.