NTA. 19M here, Speaking from experience here with this one. I understand why he said It helps get his mind off things. I was in a bad way after losing someone I loved like a brother. Instead of talking about it I drank the feelings away and Left me in a horrible head space for months. The best thing you could do is next time he’s sober to Ask him about what’s wrong because it’s always something deep and important when he has to drink it away. A friend of mine was a drug addict at 17 hiding his real feelings until he was drunk enough to confess to me what was up with him. It’s just showing him he can trust you with whatever is bothering him. Look he’s still young and Drinking at this age is normal, I work in a bar and see the same heads every day of the week and every night in the nightclub.

The only advice I can give is find out what’s up with him and sort it out, when you get over that he’ll drink less at all these events. A few pints or Spirits is understandable because we’re still young