Probably not as much allergies as an overgrowth of yeast in his body. Put him on a low carbohydrate dog food and give him some probiotics to help push the yeast from his body. He may have a round of get worst before it gets better.
I have a dog who gets yeasty, I put him on low carb food, probiotics max from Natures Farmacy and half a zinc pill once a day. Clears right up. You can also put a teaspoon of natural apple cider vinegar in his food.