Usually, if I stick to the expensive foods made with human quality ingredients, then he's okay. His base food is the same everyday (it's the dry food with two teaspoons of the wet). The toppers are just a treat, and I only give him a tiny bit since he can get sick with too much.

It sounds like your has an allergy to something, probably wheat or a protein (chicken, beef, lamb). Those sorts of allergies are fairly common in dogs, but usually to a lesser degree. Poor guy! I can't imagine how much it hurts to have blisters all over. Ferguson is somewhat lactose intolerant. He can handle a pinch of cheese, but if I give him anymore, then he gets sick.