I used to use GameMaker when I was exactly 9 so I would definitely recommend it. Scratch always seemed like it was not at all made to acheive any sort of playable game but GameMaker has a low barrier to entry but can make real stuff. I would try my best to convince him that he might have to learn to make something scaled down and 2D first, and as others have suggested, talk to him about his bigger vision game and brainstorm it for the future. That was definitely what I was doing at that time - making simple stuff and really enjoying actively dreaming about the bigger stuff.