There can be a trauma to the spine from the strangest causes. Even simple arthritis can cause what you are seeing.
I would go ahead and see the vet with him as soon as you can so that you can get him started on an anti-inflammatory and get some x-rays to find out what's going on.
The sooner you can see the vet, the better.
Let me know if you still have specific questions. I would be happy to discuss this more.