Tips on allergies

I have three english bulldogs. One’s got these little sores all over his sides. They’re these knotty bumps (almost look like bee stings) that feel like dried puss. Then they start to flake off turning into raw sores and pulling the hair out. According to the vet, it’s an allergy to something. I can give him cortisone pills which helps a lot, and the antibiotics help clear him up, but as soon as he goes off them, he starts to break out again. It used to be just in the late spring/early summer, but it’s been going on for several months now. He licks his feet because the skin bothers him, but he licks so much that skin gets raw and inflamed. Any suggestions or comments would be appreciated.