I personally do not trust Purina as a company to feed my dog, and wouldn't feed their food to either of my pups. I seriously doubt your babies problem is quality of food if you are feeding Canidae. It is one of the highest quality organic foods on the market. Many vets haven't heard of it because it isn't a huge massive marketing company like many others that are out there. I've had both of mine on it for some time now and I have only seen good things come from it
Perhaps you could try switching the flavor, they have three different kinds, maybe if he is allergic to a particular protien source it could be causing other problems to flare up. I know their Lamb formula is great for those that have food allergies.
Hope everything works out for your baby!