I would definitely take him to another vet
Have an x-ray done for sure, but if it is a disc issue in the back, it will probably not be visible on an x-ray either. My Layla has IVDD, showed many of the same symptoms, and was paralyzed within a day of the onset of symptoms. For this disease, there does not have to be any "injury" because it is just discs aging more rapidly and beginning to degenerate. A slipped, bulging, or ruptured disc is VERY painful to the dog. If there is suspicion of back pain and spinal involvement, I think this dog needs to see another vet asap.