Hi there, first off let me say how sorry I am about your dog. My sister in laws dog is going through the same thing she is a 4 yr old german shepherd. She was fine and then all of a sudden no feeling or use of her back legs. She spent two weeks in Ottawa and the tests could not find anything, no trama, tumors or injury to the spine. She was tested for every disease out there and got a clean bill of health. The only thing they thought it might be is coonhounds disease, she has had some improvement in her one hind leg but the other one seems to have lost all of its muscle mass. She has been this way for 3 months now. My SIL uses a sling to help her get around and use the bathroom and exercises her legs for her to see if they can build up some muscle, she has also changed her to a raw diet. here is a link describing this disease, I had never even heard of it before.
http://www.bobmckee.com/Client%20Inf...oneuritis.html
Good luck with your pup