My 13-y-o beagle mix experienced similar symptoms last fall, was not even able to stand for several days. Vet prescribed Vetprofen, cautioned that it might take a couple of weeks to start working. Used Gabapentin a few times when he seemed to be in the most pain. Had to help him move around for a few weeks, had a couple of setbacks, but generally got better a little each day.
Started cooking his meals, either scrambled eggs or boiled chicken and livers, and added a little canned pumpkin and plain greek yogurt to help his bowels. Started a multivitamin with Omega-3 and some glucosamine from Amazon.
After a few weeks he started acting happy again, resumed barking for his meals and trying to mug the cat beforehand. After 5 months, now, he is about 80% back to his old self. Yesterday, he was bouncing and running circles around me wanting to play. A couple of days earlier he was chasing a rabbit across the pasture (with no ill effects for him or the rabbit).
I'm no expert, but the change has been nothing short of miraculous. I hope you will at least consult with the vet (or perhaps another one) to see if there might be medication you can try. I spent quite a few days in tears until he started to improve, but I'm thankful that neither of us gave up.
My heart goes out to you.
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