One day daddy randomly has a tail for some reason. Yeah, that would probably be the way Kiyoshi found out. There aren't very many other ways for him to figure it out unless he meets them at the same time and hears how old he is and figures things out from there. Do you have any preference for how long they've known they're father and son, and how well they know each other?

Kiyoshi would be totally fine with that. In the first place he likes a little less formality, and prefers when people he knows calls him Kii-chan rather than Kiyoshi, so if you decide to have him do that it's sort of fitting, even.