These are some of my all time favorite games, that could still probably be bought (off the top of my head, will probably forget some)
RPG's to Buy:
Baldurs Gate I & II
Deus Ex (shooter/rpg hybrid. really fun)
Planescape Torment (A bit older, but perhaps my favorite)
Icewind Dale
RTS's To Buy:
Homeworld and Homeworld: Cateclysm (sp?)(still blows me away)
Red Alert 2
Ground Control
Age of Empires 2
Shogun:Total War
Sacrifice
Myth II: SOulblighter
Turn Based Strategy (warning... if he even remotly likes this genre, really addictive! Has the "just one more turn" syndrome):
Civilization II
Alpha Centauri
Hero's of Might and Magic (I, II, and III)
Simulation:
Freespace 2 (one of my favorite games ever. Space Sim)
Janes F/A-18 Simulator (solid)
FPS:
Half-Life (a must buy. You get WAAAAYYYYY more than your moneys worth cause of the sh*tload of mods out there. The best mods are Day of Defeat, Counter-Strike, Firearms, Front-Line Force, and Team Fortress Classic (in that order, in my opinion). There are also other great single player add-ons).
Unreal Tournament
Quake 3 (only if you like fast deathmatch. I actually don't like that much, but the graphics are the best).
System Shock 2
Thief 2 (not a shooter, but first person. I would give this the vote for best game nobody played).
Rainbow 6
Other Solid Games:
SimCity 3000
The Sims (I don't like it much, but it is well designed and plenty of other people love it).
Railroad Tycoon II
Rollercoaster Tycoon
Evercrack (excellent game. DOn't recommend buying as it takes over your life)
This is off the top of my head. Hope this gives you some good ideas.