I have a three year old German Shepherd that has digestive problems. He is undersized and he goes through weight cycles, anywhere from 65-80 lbs. The weight is not a concern to me, but the cycling is. Is there anything that I can do to alter his diet to help lesen the effects of this cycle? (His food allergy is corn.)

Optional Information:
Age: 3; Male; Breed: German Shepherd (White)

Already Tried:
Multiple foods, Science Diet, Prescription foods, Royal Canin and all the high end foods... Royal Canin sits best with his digestive system, but weight still cycles...