“That is never acceptable morally or halachically”
Never? Many poskim would disagree with you regarding a certain day called Purim.
Anyway, in a sense Toi is right, assuming they are not getting dangerously or disgustingly drunk. Jump on Toi all you want, but Yeshiva guys drinking on Friday night IS a lot better than what is out there among some of the world’s other youth. I know when I was in Yeshiva (and I’m talking within the last decade), there was always a little drinking Friday night, and nobody really got drunk (although some of the non-socially adept certainly pretended to). If this is the worse thing the OP’s son is doing, (s)he should be thankful. Is it ideal? Probably not. But guess what – kids will be kids.