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There are a few causes that come to mind. The first is vestibular disease. This will cause stroke-like symptoms and the dog will seem disoriented but still eat, drink and function. He might hold his head at an odd angle or walk in circles.
The next thing that comes to mind is a back or disc problem.
The third, and least decisive, is that the dog has a problem that comes with age.
No matter what, you need to see your vet tomorrow.
I am very, very sorry for your loss. I am glad that you were able to see the vet and get some relief for him.