littlejo - I agree with you that it is totally not on for people to be supplying kids with drink on a residential trip. The team manager who provided it is bloody lucky that nothing too bad happened such as one of them drinking too much and ending up needing his stomach pumped.
Apart from anything else, I do not believe that teenagers who are away to play rugby and probably do other activities in the evenings will actually enjoy themselves properly if they are too busy getting drunk. When I was 15 our year group went on a trip to an amusement park. A few of the lads had got really drunk the night before at someone's house, so at least one of them was sick in the coach on the way there, more than once. Most of them went on one ride and felt so awful that they didn't go on anything else for the rest of the day. The alcohol totally ruined the trip for them. Same thing happened at our leavers' disco. They got drunk before we went so spent the night outside puking in the bushes.
I also disagree with those who seem to be implying that it is normal for 15 year olds to get drunk regularly. Well, if it is normal then it shouldn't be. My young relative started to be allowed alcohol at home at about 13 or 14. Now 16 he is regularly blind drunk and has to be physically dragged home by his friends, or his parents if his friends have had the sense to call them. There has been at least one occasion (when he was 15) that I know of that he really should have had his stomach pumped at hospital, but his parents didn't take him because they were worried they might get into trouble over it. Instead they sat up with him overnight while he was unconcsious to check he didn't choke on his own vomit. His parents seem to think this is quite normal for a teenage 'lad' and aren't too concerned. I, on the other hand, am horrified and hope it doesn't last too long before he sees that alcohol isn't the answer to everything.
My brother and I were brought up knowing that when underage if we came home drunk our parents would be furious. Therefore, we didn't get drunk. I'm not saying that a drop never passed our lips, but we knew not to get drunk. A lot of kids have parents who think it's normal, and so therefore the kids drink and get drunk. And it's not just the drink that is the problem. When drunk they are far more likely to do other risky behaviour - drugs, unprotected sex, etc.
As for your son's situation, it is difficult as you obviously don't want your son to be singled out. If it had happened on a school trip I would say complain to the head, as it would be dealt with very seriously, but I don't really know much about the culture of rugby teams, and it might not be seen as a big deal by the adults involved.