Welcome,
iluvgizmo!
According to other posters here, diet dog foods are mostly filler. I guess I'm going to ask the obvious: have you considered changing up his diet and exercise? On exercise, could you increase either the time or intensity?
One possibility is to have him wear a small backpack on walks. I also play fetch with my dog on a hill to make it more tiring for him. You could also use one of those things that extends the range of your throw - those throw-sticky thingamaobs.
