Are you actually seeing him chewing on his tail? If so it could be pain in that area, a vet visit isn't a bad idea to rule out an injury. If you don't actually see him chewing it though, I know it's common for chins to stick their tail out of the cage between the bars and can end up rubbing the fur off the tail like that over time.
With ringworm the skin will look red and flaky. A bacterial infection I would think red and maybe swollen, maybe even blood or pus coming out. An injury to the lower spine or tail can cause discomfort and may or may not look any different from normal though.