I would err on the side of caution too in this scenario. But I wouldn't condemn you if you chose to trust him. I allow my nearly 13yo to walk to school (most parents don't) and I allow my 4yo to sip wine at the table... but I wouldn't allow my son to go to a party at night where there are older kids drinking. Alcohol affects each person differently (I lived in a pub when I was 17/18/19). How can you tell if there will be another 17 yo there who might get violent? But ultimately it's your call, you know the kids involved, just as long as the party isn't gate crashed I guess.