My dog has lost use of his hind legs?

He is a beagle mix that we rescued about three years ago. He was already old and he already had some back problems and arthritis in his legs. Actually he walked with his back to one side. But while we had him that straightened out quite a bit but unfortunately we kind of felt sorry for him and he put on a lot of weight while we had him around 17 pounds. He had a low thyroid test come back so he has been on thyroid medicine and also tramadol for pain. So now in the last four months he started losing the ability to walk. He has incontinence problems and we are seeing a problem peeing now as well when we do take him out he has to make a greater effort to pee. We have asked the vet to find out when is the time to put him down but she has been evasive and asked us to take him to a orthopedic specialist. We do not have the money to start a new course of treatment for this animal as we have other animals in the house now that need medical attention too. What should we do? He still drinks and eats anything and everything. He has eye growths and now he wakes up with really goopy eyes as well unless I remember to medicate his eyes. I ust think we are not doing him a favor keeping him around. Anyone with any suggestions?