This could be a serious spinal issue — you need to take him to a vet for evaluation. Swelling disks, a pinched nerve, could be the problem, but you can’t be sure what is going on without a vet’s evaluation and probably an xray or soft-tissue scan. Any thing involving the spine is pretty serious in my book, so I wouldn’t wait. Often problems can be resolved with crate rest and some medication for swelling and pain.
Good luck!
And strangely this ended up being a question instead of a comment/reply. I think it might be a glitch.