Leave the poor "Weight challenged" boy alone it is not his fault that he has got the problems that he has, after all by bullying him you might be making the poor boy fatter, because he comfort eats, and you guys laugh when he gets stuck in a door way, but won't it be better to help[ him though the door by kicking him up the arse, or a crowbar.
You lauhg when he comes last in races, but do him a favour stand at the end of the finishing line and wave a bar of chocolate or tub of lard so he reaches the finishing line first.
What I am saying is give the poor boy who is after all just big boned a break, becoming friends with him and give him a lift (not literally, you would break your back)