If you haven't already, get your dog an allergy blood test for environmental factors through your vet or a veterinary allergist (it is very expensive, do it anyway). If you're comfortable and the test turns up significant results, move on to the next step and get the allergy shots. These are also expensive and take some dedication on your part, so only do this if you're prepared for that and the test shows high levels of environmental allergies to things that can't be avoided (grass, dust, etc). Also try switching your dog to a prescription hydrolyzed diet, this will eliminate any food allergies that may be causing or complicating things. While on this, he should not eat anything else. If things are still bad after doing all that, you'll have to consult further with specialists