Hi... depends a lot on his interest but..
Out now. Consider:
1) Half Life 2 Orange Box ... FPS ... fantastic value (contains five separate games and all for around £26).
2) Bioshock ... FPS
3) Stalker ... FPS with some RPG elements. Should have been game of the year in my opinion.
Command and Conquer 3 ... RTS
4) Tomb Raider Anniversary ... Third person adventure (Lara still has some life left in her).
5) SWAT 4 ... Tactical shooter ... on budget label now. Still a great game.
6) GTA San Andreas ... American third person shooter in a simulated real world with RPG elements (!!!)
7) Oblivion ... RPG/FPS hybrid. You could loose yourself in this one.
Coming before Christmas.
8)Crysis ... FPS ... should be a cracker.
9)Call of Duty 4 ... FPS
There should be upwards of a hundred hours game play in that lot.
My choice would be: 1, 2, 3, 4, 7
Though I'm sure this sort of choice could be debated forever.