Um I don't know what else to add, he was already like that when I bought him yesterday. You can see in the first picture how his anal fin is kind of wavy at the back. I don't know if that's normal or not.
That looks more like a genetic deformity or some kind of old injury. It could have been something genetic that he inherited, some damage he took maybe from a sibling when he was young, or perhaps some not so great conditions from his time with the breeder/supplier caused his rays to grow a little crooked.
Impossible to say which exactly for sure, but regardless its nothing to be concerned about. Its just the way his fins are.