1. Portal. Short but absolutely amazing.
2. Torchlight is extremely easy to learn and fun to play.
3. Half Life 1 & 2 are excellent story driven games without excessive gore but does have mild use of language (only on occasion, and it's nothing more than sh*t or damn)
4. The Penumbra series. Best horror games ever. Very little combat, the focus is on getting past monsters /puzzles without getting caught and killed. Awesome fun game.
5. Prey is good, has lots of obscenities but there is a game option to turn off the cursing.
6. Civilization IV. I've probably played Civ IV more than any other game... ever. Best strategy game of all time. Heroes of Might and Magic III is a close second.
7. Plants vs. Zombies and/or Defense Grid - the two best tower defense strategy games on PC.
Plenty of variety in those recommendations, and Between Half-Life 2, Plants vs. Zombies, and Civ IV he'll get a ridiculous amount of play time for dirt cheap.