This ! Learning to code will come after, spending time with your son writing down ideas might be more fun at first and it's a good time to teach him that games are thoughts first and then coded after.

I would have recommended Scratch [1] for a first introduction instead of hoping into code right away, but since he is 9yo he will most likely want to hop on big game engine like he sees his favorite youtubers doing.

so while you guys are thinking about ideas, you should look up and compare engine in this thread and learn one, then teach him and make the game/prototype together later.

[1] - https://scratch.mit.edu/