To be honest, I think they are missing something and sitting around and waiting for a treatment to work for a disease you don't even know if he has...
You need to request a referral. I suggest a neurologist. YOu need to have it determined where in the neurological pathway your dog is having difficulty. Knowing the breed there may be a partial disc protrusion causing what you are seeing and the only reason there is some recovery is that he has been taking it easy.
Xrays don't always show anything...that is why I think you need to go the next step. A very large population of dogs in N.England do have positive titers for Lymes but do not have the active disease.
I really don't think it is a mystery illness, I just think you need to get a referral to the next level of vet.
Let me know if you have further questions.
Dr.V
Dr. D.A. Vamvakias
Veterinarian