From the pictures, personally I'd vote against fin rot. To me, it looks as if his fins have just grown out, which is very normal for any betta over time, and perhaps he took a small nip or two that healed up very quickly, which would cause the wonky ray thing going on there. Or it could have just grown out like that on its own. IME, sometimes rays don't always grow out straight, especially in fuller/bigger finned mates.
Has it been looking as if, over the past few days, his fins have been slowly getting shorter though?