The only thing i can think of would be restricting of his diet, tess cooks book says for over weight turtles to feed every four days, but if he is outdoors then he will eat any and every bug he can find im sure so it may not work as well.
He lives in a pen with two females who need to eat. I don't want to set him up with a new pen. Maybe I'll go with the treadmill! If I put a female at the other end, maybe that would work.
I'll try those new Jenny O turtle pellets.