Retrievers are prone to this. You could also try ear drops containing aloe vera and calendula. The aloe is good for inflamation, allergies and healing the ears and the calendula has anti-bacterial and anti-fungal properties. I am selling nothing and I have no idea if these work or not, but here is a link to some drops that contain calendula and you can buy aloe vera at any health food store.
You said you tried changing the food from one commercial brand to another. Have you considered trying to switch him to a totally raw food meaty bones diet? This may address a potential underlying food allergy to commercial foods. A raw food diet will boost his immune system in a big way.