Should l let my son go to his dads? he let him drink at 15. I am not happy, but dont know what to do.?

We are not together, he only sees him in the summer time. At a family gathering he allowed my son to drink a beer just this week. Aunts, Uncles, Cousins, and grandparents did nothing about it. Actually i heard they said it was ok. I am completely and utterly against this and think that we should be teaching my son to abide by the law. He also ways like 100 pounds it would take much for there to get alcohol poisoning. Under age drinking is against the law and could lead to more. But i am torn because it is the only time he gets to see his dad and unless i decided to never let him see his dad again there is no point in me saying he cant go now. What is the difference from this summer as opposed to next summer. They both promise it wont happen again but have showed horrible judgment. My ex doesn't even feel it is a problem and neither does his family, what am i saying if i send him. That it is OK to do what ever if i am not around. I don't know, i want him to have a relationship with his dad but need to be a parent at the same time. Advice would be helpful. Please