Modelling: Blender is a good start. There are tons of tutorials on the subject. With blender you can creat backdrops, models and animations for games

Game Engine: I would nowadays start with Godot, there are equally many tutorials

Programming: I recommend doing something using processing.org, this is a simple IDE with a powerful library that can be programmed using Python/Javascript/Java. This is one of the easiest ways to draw something in a window using code.

Edit: I think the most important thing is that you go through this together with him. The way is much more important than the goal. If you do this right, you will most important) spend time with your kid, and teach them how to tackle a thing that even daddy isn't sure about doing. Discover things together with your son, let him try things even if you already know better. The right motivation is much more important here than getting to something good.

And it is hard to say remotely which part about games motivates your kid. Maybe it is the visuals, maybe it is the programming (this is more rare), maybe it is the result. If it is the latter a specialized 2D-engine would be a good start.