We have the same problem with our tri. He is allergic to Wheat, Soy, just about everything. We feed him Blue Buffalo Allergen mix which is lamb & rice which seems to work the best for him(Most allergy mixes are Lamb & Rice). We found the so called Allergy food he use to eat had soy in it, he had problems with it. You have to read the labels of everything he eats.
We give him half a Benidril if he has an outbreak. Our vet said the spots are actually a staff infection. During an outbreak we put him on antibiotics, or but a little Neosporin on it (not too much they lick it).
Our dog is also allergic to grass. Bathing him once a week with oatmeal shampoo when he does not have a rash, then a medicated shampoo if he does have a rash. The medicated shampoo we put on him, then take him for a 15 minute walk ( In warm weather)- then rinse him after the walk.
This combination has pretty much eliminated the issue, also would get Diarrhea & bubbling stomach. No, he has firm stools no rash, doing great.