Ingredients below from their US website, it also contains corn syrup, maybe he reacts to corn?
Unflavoured:
Corn Syrup Solids, Fractionated Coconut Oil, Canola Oil, High Oleic Safflower Oil, L-Arginine, L-Glutamine, L-Lysine-L-Aspartate, Tripotassium Citrate, L-Leucine, L-Phenylalanine, Dicalcium Phosphate, L-Proline, L-Valine, Glycine, L-Isoleucine, N-Acetyl L-Methionine, Tricalcium Phosphate, L-Threonine, Mono and Diglycerides, L-Histidine, L-Serine, Sodium Chloride, L-Alanine, Magnesium Acetate, Choline Bitartrate, L-Tryptophan, L-Tyrosine, Diacetyl Tartaric Acid Esters of Mono and Diglycerides, M-Inositol, L-Ascorbic Acid, L-Cystine, Propylene Glycol Alginate, Taurine, Ferrous Sulfate, L-Carnitine, Zinc Sulfate, DL-Alpha Tocopherol Acetate, Niacinamide, Calcium-D-Pantothenate, Manganese Sulfate, Cupric Sulfate, Thiamine Chloride Hydrochloride, Pyridoxine Hydrochloride, Riboflavin, Vitamin A Acetate, Folic Acid, Potassium Iodide, Chromium Chloride, Sodium Molybdate, Sodium Selenite, Phylloquinone, D-Biotin, Vitamin D3, Cyanocobalamin.