Our Quinnie, bless his little soul, had terrible allergies. Bad enough that his heart enlarged and he had a murmur. We ended up moving him to an all fish kibble diet and our vet got us allergy shots which which I administered SubQ once a month. Plus he was on a mild steroid by mouth. He did much better after that. However, we believe that the steroid masked his lymphoma which is the reason it made it to stage 5 before we knew he was sick.
You might ask your vet about the allergy shots though.