I've heard something about plain, canned pumpkin, as well. Does that have the same effect? I will try the yogurt, though, thanks! I tried weaning him slowly, but ran out of the old food and had to make the transition quicker than planned. Maybe that had an effect.

Regarding Iams, I have heard countless negative comments. To briefly name a few:

-change in formula since being taken over by Proctor & Gamble (same with Eukanuba and some others), including a lot more fillers such as corn, etc.
-giving kickbacks to vets for recommendations (not sure of the validity of that, though)
-animal testing!!!