Back Problems with Shih-Tzus
I have had two of these babies with back issues. My vet performs a bit of chiropractic treatments, and Little Bob also gets pain killers when the problem is severe. He also takes a vitamin supplement that has helped A LOT. We are better at being able to notice when he is having an issue before it gets severe so we limit his activity by either crating or gating him in a room. He is NOT allowed to go up or down stairs at all anymore. Like your dog, he eats perfectly fine when this happens, so that isn't really an indicator of the problem. You can just tell by watching how he moves.
Good luck with your doggie. I wouldn't trade mine for the world, and would do anything to help him.