Allergies are a tough one. Like Claporte said try giving him something he has never had before and stay away from ingredients that say "bi-product".
Considering where we live it could be dry skin too. The omegas will help with that.
FYI-We just had to take one of our dogs to the vet for vomiting and diarrhea. The vet ran some tests and thinks it is salmonella poisoning. We feed Purina, which wasn't on the recall, but Purina One Cat food was.
Yadlim, you posted the same link I was going to post.