As the link that Patricia posted states, allergy tests are very inaccurate. Unfortunately, an elimination diet is the best way to determine the problem ingredients. You would need to feed a prescription hydrolyzed diet to clear his symptoms and then gradually add a protein one at a time to see if there is a reaction. The hard part is he will not be able to eat anything else during the trial.
Rx diets can be very useful! I have a cat with presumed IBD on one. He’s doing awesome! Another, on an Rx urinary diet that saved his life. Good luck!
