I have to point out that if your son has a Mac, then all hope is pretty much lost…most Mac First Person Shooters have no editing programs available for them. 
:mad:
If you’re not too conserned about your son being affected in a bad way by violent video games* (and you have a DVD drive in your computer), you should pick up the Special Edition of Unreal Tournament 2004. It comes with a bonus disk containing well over 100 hours of “Video Traning Modules” teaching about creating custom material. Also, the Unreal engine is quite popular, so the knowledge gained can be useful when dealing with several other games, too.
*That stands for almost every single FPS, though.
I’ll third that reccomendation for Unreal Tournament 2004 special edition if shooters is your son’s fave genre.
Again, let me restate, you’ll NEED those video tutorials, 3D graphic development, and mod development for these games is NOT very easy at all, it’s going to take some commitment on your son’s part. Hopefully he’ll come out of it with some real world skills he can use in the future though 
Good luck.
Pleonast is me, the game I recommended is Nethack.