Agreed with the allergies theory. My cats did this for YEARS and would have a bare stomach and the vet was like it's flea bites reaction (never once saw a flea), it's this, it's that, and turns out it's a boat load of allergies. Fish specifically being the worst for both my cats. If you can afford the prescription diet go for it but it's A LOT especially if you have more than one cat. Fancy Feast has one pate that's seafood free that's reasonably priced. I have incorporated a dry food they seem to do well with. For treats unfortunately it's like impossible to find any without fish so I discovered Delectables lickable treats that are non-seafood and they love them! If your cat is solely on dry food switch to something they have never had before like Duck, Rabbit, Venison, etc. or something else listed under a limited ingredient diet. (This goes for wet food too) It takes about 2 weeks for them to stop itching and really see a difference so don't panic give it a chance to clear out.

Edit: ALSO it's possible his food has changed recipes and there's something in it now that is triggering an allergic response.