Responses
I don't think he was ever truly "attracted" to the kid. More like, the kid's youthful feeling. Campanella had a passion and vigor that he didn't have anymore. He missed that feeling, since he last felt that way when he was with his ex.
He was attracted to his boss the whole time, and vice versa, for a longer time than the manga showed us. (Or, at least physically attracted)
Was it the age difference?? The number of conversations?? Like if had spent equal amount of time with the high schooler would his choice have changed?