A. Dogs can get arthritis in their spine as well. It is quite common, in some breeds more that others, but can effect mobility. I would suspect is is something like this, unless it is a lump/tumor. I would have it checked out by your vet
Any dog that has arthritis should have a consult with their vet to discuss pain management.
I would also recommend getting him on a glucosamine supplement if he isn't already. You will need to wean him on to it over a week or so, up to the full dosage and feed with food to avoid tummy upset.
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