I think you should consider consulting a Veterinary Internal Medicine Specialist if you feel your veterinarian has not been helpful, otherwise I would go by his recommendations.
Supplements are not medication, often they can cause harm. They tend to be expensive, the same money could be spent on getting professional treatment, a correct diagnosis and effective treatment.
PS: I would be very leery to take advice from anonymous strangers on the internet that may or may not have a medical background before listening to a veterinarian that has examined the dog and knows it’s history.
