My dog has a rash!!!!!!!!!!?

I had a dog that lost all his hair on his rear every summer and come winter is came in beautiful. After repeated trips to the vet, we decided to try another vet. They told me he had eczema, suggested using Bag Balm or Udder er on it and treating him often for fleas, as he also had an allergy to flea poop. That's why his hair would grow back in the winter, cause the fleas would die off. I understand how frustrating this can be when you take the to the vet repeatedly and don't get a solution to the problem.