FOSS alternatives to Unity exist though. And from my personal experience it looks like Godot seems like the better engine anyways. Not to mention the fact that there is no need for a game engine to create a game. Opengl + a windowing/utility library is ideal.
All the others have great suggestions but I’d just like to add there many open source projects out there and given example code of how something is done and letting him play with it can be tremendously helpful. For example finding some simple 3d programs and lettting him change the source code to do whatever he wants or just giving him a template like https://editor.p5js.org/1alimaze/sketches/IJpxIEME8 which I made when I was first learning and letting him play with the values and add more objects.