Your situation is very tough because you want to encourage this but also you have to be realistic and that's what your kid probably doesn't want to hear.

I always try to get across to my daughter of seven that you can't scale a mountain in one step just as you can't eat a cake in one bite and that trying to do that is guaranteed to be a miserable experience that's going to put you off the activity forever.

I don't think I've quite managed this yet but if I was you I would try to convey that a 3D game is a master level achievement that, while totally possible eventually, you simply have to work up to. And then maybe somehow pivot him into Scratch, Gamemaker or other such tools where he can start learning in a playful and fun way.