I've never been on RCCL but I've been on Carnival and I loved Club o2 on CCL. It seems like there would be plenty of activities that he would enjoy too. They have a teen club where most teens dance at night and during the day they have video games and things like that and other fun activities. They also have basketball games and volleyball games and things like that.
And for the twelve year old there is plenty of swimming on any cruise ship you might pick. They all have 3 or 4 pools. And I know that Camp Carnival does have a sleep over one night where the kids can stay at Camp Carnival all night and then leave at like 8 am and they do fun things and watch movies and things like that. I think there was a fee involved there but it wasnt too much and all the kids that I know that did it loved it and had a lot of fun.