My adventure with game engines also started out in GameMaker as a kid. I was eventually drawn to its 3-D drawing abilities and, finding them very lacking, started looking for proper 3-D engines. That’s how I eventually ended up at Panda3D. 
GameMaker is great for starting out with some simple 2-D games, but your son may find himself wanting more if he finds himself being drawn to 3-D; in which case he’d be more than welcome here.