Allergic to chicken ???

Our Pit was 80# and I did everything to get him to lose weight. After low cal food, green beans & pumpkin he lost a few pounds. I gave up. Then he dropped down to 70# and looked good. Then five more and I took him to the Vet Monday. Something's not right. On previous visits he had an X-ray for old dog lung which was relieved and bloodwork that came out ok. No reason for weight loss but possible "tumor" so an ultra-scan was ordered. Happily no tumors but found a thickened (I think was the word)intestinal wall. The thought was that the food was going through without being absorbed, causing periodic diarrhea and weight loss. I feed them Blue Buffalo with chicken as the first ingredient and mix in boiled chicken. The only thing the Vet can think of, he's allergic to the chicken protein. It's irritating the intestine and causing the weight loss. I'm getting the BB basic tomorrow and topping with beef, no more chicken. The Rotti has "rumbling" stomach every week or so as does the Pit. Nothing bothers the other dog. Maybe the Rotti has the same chicken problem. I understand chicken allergies are common and wondered if your dogs had any experience with it and if so what were the symptoms? I always thought allergies caused them to lick their feet as the Rottweiler does occasionally........ Thanks for any replies.