I agree with Jacque. Little big planet is probably the most accessible game for "creating games". It's for PS3. Also, Torchlight for PC comes with the editor that the company used to make the game. He can use the assets (props and such) that they used for the actual game and make his own levels and stuff. Or make new…
This is a great idea and I second this. The starcraft 2 level editor, for example, has so many possibilities(you can make entirely different games) and if he likes it he could get into more advanced programming and what not. Start with the basics!