i had a black lab who was 5 years,lost alot of fur, and vet couldnt find anything, and just kept getting antibiotics, after couple of months he was caughing, and did later find out it was due to his immune system not working, and resulted to this and his lymp nodes were very swollen.
I did try to post the question to Dr. Cheng but that Forum seems to be full!
Your vet seems to have ruled out anything I could think of, but it may be interesting to see if it turns out to be fungus. A neighbor's dog has the same problem with massive hair loss due to recurring fungal infections. It's just something his owners have to stay on top of, but not serious. It also didn't crop up until he was 6 years old.
Have you posted your question on the Ask a Vet forum? Dr. Cheng would probably have some other ideas to run by your vet, along with some dietary aids that might help.
His diet has not changed he has been on the same food for several years. He also does not have fleas and the vet did a flea comb and did not find anything with it either.