What I find interesting about your post is that your son is designing on paper. He may be a Game Designer at heart and not a technologist, so you'll have to tease that out. The difference would be whether what he really wants is to learn to build, or to have his thing built.
As a Game Designer, he'll be more interested in the rules, the world, the story, the Game as it lives in his mind.
As a Game Developer, he'll be more interested in the tools, the technology, the experience of playing it on the computer, and hopefully the struggle of iteration and building!
There is a high likelihood that, at 9 years old, what he is envisioning is totally unrealistic for you guys to attempt to build. You are cursed with the skills of a competent developer who is up for a challenge, so you might think it's reasonable to attempt. I'd be careful here. This isn't up to you to "not let him down". Sure, you're Dad and you're awesome, but even John Carmack would hesitate to guarantee he could make good on a 9 year old's first game design. :)
It also sounds like you're unfamiliar with game tech, so this will be a learning experience for you. Are you up for it for its own sake? Or are you just trying to make good on your son's vision? That is a big hill to climb to just have your son look at what you made and go "meh".
So if you think it's interesting to try out, and you guys want to give it a go, I still think the best starter pack for gamedev is Tom Francis's series on Unity on Youtube called "Make A Game In Unity With No Experience".
https://www.youtube.com/playlist?list=PLUtKzyIe0aB3TZfe2wsIg...
Mind you, this is a 3 year old series. Unity is an evolving platform. Stick to what Tom is using and you'll be okay.
