How old is ruckus? If its ringworm its more than likely not from the food. Most allergies develop with age like seasonal allergies but food allergies are usually in the first 2 weeks if not sooner. HAve you tried washing him with selsun blue? Aslo a high sulfur cream would be great for the spot(even if it is ringworm). If the food isnt causing him problems I wouldnt change but if it is a food issue then I would change gradually to prevent upset.