Poor little thing...I hope he feeling better soon.
It may be time for a second opinion. Twice I have taken my dog to specialists because his regular vet just didn't have the experience and equipment to deal with his issues.
It was very expensive (spent $5K this past summer!) but better than $200 here, $100 there and him not getting any better.
Because I have a poorly bred Rottweiler I knew his medical costs could be very high so I opened a savings account expressly for medical...saved my butt twice.
Mom to the late Cyrus the Fluffweiler CGC forever in my heart and James the Alpha cat