Options to treat allergies are to continue to treat symptoms with medications (okay if symptoms only occur occasionally
and your cat only needs 1-2 steroid
injections per year), or to identify
and treat the underlying cause. There is no accurate skin or blood test for food allergy in pets, the test and treatment are the hypoallergenic diet trial (rabbit/pea, duck/pea, venison/pea, z/d are all good options, whatever he'll eat) and nothing else for 8 weeks.