My lab/mastiff baby has allergies, to everything. Try fish oil, if not already using it. And over the counter allergy meds. Mosby and I both take Claritin in the spring. :-) However, if you think it's food related it will be a trial and error with new foods. Is it to food or something else?
I’ve been told great things about K-9 Immunity by K-9 Medicinals. It is evidently great for dogs with existing issues or as a preventive. You can find it online and read why it is a pretty unique product in the market.