Looks like you got a kinked tail. Its all normal, do you know if he's had it since birth? It happened to my guy but I was never sure if it was cause he almost had it nipped off when growing up with siblings around or if it was something that occurred after being in my care. But like you said he still used it fine without any problems so I was never worried about it. If you search for kinked tail you'll find multiple threads about it.