I'd definitely try out Canidae and see how he does. I've heard and seen of a lot more good things happening for dogs on Canidae than I have bad. Winston is doing incredibly well on it - it's one of only two foods he can eat without getting sick. I'd stay away from Royal Canin. The ingredients aren't that great and it's way too high in price for the quality. And yes, I'd stay away from senior food as well.