Hi :
Ouch!!!!! lota money and no relief in sight????
Have you tried another vet. Your baby doesn't sound like he has anything very rear. If it was and I hate to say this I would think the prob would be getting worse instead of staying the same. But by the same token it should be clearing up. I would definitely try another vet or animal hospital where they have some specialists that can prescribe some sort of topical for him.
Also dogs get what is called hot spots. That means they are always licking the same spot and this causes a skin disturbance. That might also cause him to bite or scratch the spot. But it sounds like he just has a rash of some sort.
I just had my little female Yorkie with a growth on her tail that the DUMB vet diagnosed as a tumor....it turned out it was a skin thing form some shampoo I used on her and it started to clear up the very day I put the topical on it. I would explain what had happened to her but it is more than you want to hear right now.
I would definitely try a new vet that knows a little more than this guy/girl and will help you with out all this discomfort to you dog and expense to you.
Tina