Instead of telling him he needs to play all nine games straight up (because that can be a daunting prerequisite for anyone), try this:
Start off trying to talk him into just playing the first game. He can test the waters with that one first of all, and if he likes that, then hopefully he'll be hooked, at which point he probably won't mind playing many of the other games. That might be more palatable to him than telling him he has to play all 9 (technically, it's actually 12) games before KH3. That makes it sound like a university course. Also, if he ends up not liking KH1, that'll probably tell him all that he needs to know for 3.
Keep in mind, he'll probably just do whatever he wants to do. If he wants to start with 3, that's his prerogative, even if he probably won't get hardly anything out of it. In a perfect world, we'd be able to just make people understand how necessary it is to play all of the games in order to get the real experience out of it, but many people just aren't that patient or willing, and gamers aren't really used to having their entertainment demand so much of them. Try to explain that jumping straight to KH3 would be like watching Avengers: Infinity War without seeing any of the other MCU movies, and maybe that might make him understand. But otherwise, there's only so much you can do with someone dead set on doing things the most expedient, if erroneous, way possible.