What really helped us was switching to a more raw and home made type diet. After lots of trials my routine is to feed The Honest Kitchen dehydrated raw and change it up with some prepared frozen raw like Nature's Variety. Just going with THK and half kibble cleared many issues up, after a couple months I eventually stopped kibble.
If this does not help, then time for $500 environmental allergy blood test.
Hope your dog feels comfortable soon!
I forgot to mention, it did not matter what protein source we used, just changing to a fresher, less processed diet worked.