dog has no movement of entire back end

[Julie]

Our family dog has quickly lost all movent in his back end (hips, back legs, tail). This seemed to come on quickly. He is 12 years old. We are not sure what breed he is exactly, but we know that it is a mixture of lab - possibly german shepard - maybe even collie?? He has been to the Vet ( 2 in fact) and both have said that they don't know what it is. I don't see how these symtoms can't be given a name. It would just make more sense to us if the Vet could say that is was arthritits or something. He was so active and healthy now he can't even stand up on his own. He has to be lifted up to his bowl to eat and carried outside to do his business, which is difficult as he seems to be losing all feeling in his back legs. One leg even looks as though it cramps up or something as the paw curls when he stands. It does seem as though we have to make a decision, we know, but we just hoped that maybe someone had an idea what this might be and if there are any possiblities to fix this! We never thought this process would be so difficult. Can someone help??
Thank you

Julie