How to to keep my sim on sims 3 skinny and not fat?

ok so i adopted this kid and now his and grown up my idea for him was to be athletic and fit looking hard cord sports guy but he always gets fat and iv tried the body shape changing thing he gos back to normal. I've tried testingcheatsenabled shift chick him and edit sim in create and sim and put im back to skinny and all the way buff and ripped, his master athletic skill and i have fast metabolism on him but nothing stays the next thing he does it gos right back to fat (literally if he opens a book the fat comes right back not just eating) idk what esle too its really annoying if anybody got any other ideas that might work let me know plz :)