[Jotaro stares blankly at Joseph and is quiet for a good, long moment.]

Call me JoJo Jr and I will keep calling you 'gramps'.

[Even though his face doesn't show it, he get his grandfather is being genuine. At least he's easy enough to read when it comes to that.]

Take turns to train them and occasionally make them train together. It'll be good for them to work as a team. Who knows, maybe Rush may cooperate more if he sees how well training with Mitzi goes.