It may be the soybean products or flaxseed;
Lamb & Rice Large Breed
Lamb Meal, Cracked Pearled Barley, Animal Fat
(preserved with mixed tocopherols and citric acid), Chicken Liver Flavor,
Soybean Oil, Dried Beet Pulp, Potassium Chloride, Flaxseed, Iodized Salt,
Vitamin E Supplement, Choline Chloride, L-Lysine, vitamins
(L-Ascorbyl-2-Polyphosphate (source of vitamin C), Vitamin E Supplement,
Vitamin B12 Supplement, Vitamin A Supplement, Thiamine Mononitrate, Riboflavin,
Pyridoxine Hydrochloride, Niacin, Folic Acid, Calcium Pantothenate, Biotin,
Vitamin D3 Supplement), Taurine, Glucosamine Hydrochloride, Chondroitin
Sulfate, minerals (Ferrous Sulfate, Zinc Oxide, Copper Sulfate, Manganous
Oxide, Calcium Iodate, Sodium Meal, Brewers Rice, Rice Flour, Ground Whole
Grain Wheat, Ground Whole Grain Sorghum, Corn Gluten Selenite), L-Carnitine,
preserved with Mixed Tocopherols and Citric Acid, Beta-Carotene, Rosemary
Extract.
Regular Large Breed;
Ground Whole Grain Corn, Chicken By-Product
Meal, Soybean Meal, Animal Fat (preserved with mixed tocopherols and citric
acid), Soybean Oil, Chicken Liver Flavor, Dried Egg Product, Flaxseed, Iodized
Salt, vitamins (L-Ascorbyl-2-Polyphosphate (source of vitamin C), Vitamin E
Supplement, Niacin, Thiamine Mononitrate, Vitamin A Supplement, Calcium
Pantothenate, Biotin, Vitamin B12 Supplement, Pyridoxine Hydrochloride,
Riboflavin, Folic Acid, Vitamin D3 Supplement), Dried Chicken Cartilage,
Choline Chloride, Vitamin E Supplement, Taurine, Potassium Chloride, preserved
with Mixed Tocopherols and Citric Acid, minerals (Ferrous Sulfate, Zinc Oxide,
Copper Sulfate, Manganous Oxide, Calcium Iodate, Sodium Selenite), L-Carnitine,
Beta -Carotene, Rosemary Extract.
Most dogs develop a food allergy to something that's been in their diet for a while; soybean oil and flaxseed were in the Lamb and Rice, and both soybean meal and oil, and flaxseed are in the regular large breed formula. You've already proven he's not allergic to chicken.
Life isn't about waiting through the storm; it's about learning to dance in the rain.
Janice