At the end of the day, it's not up to them, it's up to you. They are probably making it sound like the grass is greener, but have they really thought this through? Probably not. Your DS would be feeling all sorts of guilt - his grandparents have probably talked it up, made it sound like it's going to be awesome, and what 15yo woudln't want part of that? But he doesn't want to let you down either. Get him to come home, go to school and then keep going there for holidays if he needs the time away from things at home.