Since no one else has mentioned this issue, by any chance has he been to Vet Dermatologist and been tested for Sebaceous Adenitis??? Has a skin biopsy been done on him???
When I worked as a Vet Tech we had a clinet whose dog was Dx'd with Sebaceous Adenitis, they did everything possible for their dog for over two years in the end he was so miserable, they let him go rather him suffer any longer with the issue, which has no cure.
Here is some info on it.
Inflammatory Skin Disease in Dogs | petMD