There are many many better foods out there than Purina...
With suspected allergies though, the starting point should be a food that contains none of the same ingredients. That's not quite as simple as it might sound, in your case - because puppy chow contains some unidentifiable ingredients that actually could be *anything* (e.g. "animal" fat, "animal" digest, "meat & bone meal").
The reason you should start with something that has no ingredients in common with the puppy chow, is so that you have a completely clean slate. If your dog indeed has some food allergies, then it could be to any one (or several) of the ingredients in his current food. The only way to find out which ingredients are problematic has 2 steps. The first of these is to eliminate every single one of those ingredients from his diet (to let him recover). The second step is then to add those ingredients (or at very least, the nutritious ones) back in to his diet one by one to discover which ones are problematic and which aren't.
I would thus make the suggestion of a food such as Natural Balance's Sweet Potato & Fish formula. It is actually aimed at allergic dogs, and has limited ingredients. An alternative could be the Potato & Duck formula of the same brand. What each of these foods has going for them is that they have NO ingredients in common with the puppy chow (and nothing that comes under the umbrella headings of "animal" or "meat").
You would need to feed whichever new food you choose for 12 weeks - without feeding anything else (including treats) - to properly prove whether or not your dog suffers from a food allergy. If all his symptoms clear up, then its a strong sign that he did have an allergy to something in the puppy chow. To discover which ingredients were the problem, you would need to add them back in one by one. If his symptoms don't disappear though - then it's a strong sign that the problem is actually something else entirely.
