Poor Buster got worse over the weekend so we took him back to the vet today. He got to the point that he couldn't take a step without stopping to scratch his belly with his back paws. Vet thinks he got into bull nettle or something. Prednisone pills were prescribed.
Meanwhile, I found a local pet store, Pet-O-Rama, that carries Innova pet food. I bought a small bag to try. No corn!
Thanks again for all the help and advice.