1) Why not try the freeware versions of the old arcades classics like Pong, Space invaders, Lunar lander, Galaxxian and so on just to get him interested.
2) I wouldn't throw him in at the deep end in "big" serious games to start with.... Have to start off very slow ans very simple.
3) .... but maybe Wow trial as it involves very quickly and the training areas are trivially easy.
4) Dare I suggest Sims
5) Dungeon Keeper 2 if you can find a copy
6) Worms, Lemmings etc
7) Something like counterstrike or unreal tournament
The theme here is mostly jump in and play with nothing heavy to learn or think about to showcase some of what is possible with PC gaming.