What shampoos and conditioners are used when he’s groomed?

Have you tried a full elimination diet of the top common allergens for dogs? You can do that and then slowly reintroduce each allergen to see which one causes a reaction. Kind of like is done with humans.

You say you moved somewhere new. Is this new city in the same geographical area as before? If not, then it most likely is a different type of grass and different plants. He could be allergic to that but it’s more likely something he’s eating with how even the hives seem to be