Have you considered taking him to a training hospital or speciality center?
- two personal examples - happened to same dog sadly
1) My local vet, who is very trustworthy and competent could not cure my dog of his corneal ulcers. I took him to a board certified opthomolgocial vet and that vet saved his eye sight
2) He developed Addisson's disease. My own vet missed the diagnosis but an emergency hospital caught it. He was then saved my a place (one of the few in New York) that specializes in criticaly ill animals. The observed him arround the clock and gave him IV fluids until he got better. The place is called "The Center for Specialized Vetrinary Medicine" and is located in Westbury Long Island.
if there is a vet school or training hospital near you, I'd take your dog there for a third opnion. These hospitals deal with very sick animals and obscure cases that local vets rarely see. They may have a greater depth of experience and will be able to get you a diagnosis (hopefully).