I am very sorry to hear of your dog's lameness. I have several suggestions as to what to do, however.
1) GET A SECOND OPINION!!! I cannot stress this enough.
It can be anything from hip/elbow displaysia to Degenerative Myopathy. Both are heart breaking, not only to the owners, but to these magnificent animals.
2) DID YOU GET THIS DOG FROM THIS BREEDER?? I plan to purchase a puppy from these people and would like to know where your dog came from.
3) What type of diet did you feed your dog? From Day 1.
4) When he was a puppy, did it have impacts of play (intense) whereby it was followed with long periods of inactivity?
This can have a major impact on the development of it's joints.
5) What type of supplements did you provide and when did you start?
6) Not a question, but a possible solution: Have you taken your dog to a vet school for diagnosis? This should have been recommended at the onset by your vet.
I hope I have helped in some way. I wish you and your dog well and hope this can be corrected.