If he doesn't have any FPS games, then I think you should only get one new game for him, and buy him more of the older, but also great FPSes of year's past.
If he likes Halo 3 then get him Halo 3 before you get ODST. That is a must for him.
Secondly, you should get him a nice story driven FPS adventure. Your two best bets are Bioshock or The Orange Box. I personally like Bioshock a bit better, but the Orange Box has a ridiculous amount of content. It's five games in one. If you don't mind you could get him both since they're easy to get for $20 each.
Lastly, get him Modern Warfare 2. Without knowing him but reading what his tastes in games are, I think he would enjoy this game a lot.
If you were planning on buying three full priced games, this would allow you to get him more for less money. You could walk out spending about $140 for all four of the suggestions I made.
Tag: Became a freaking mod and a complete douche, coincidentally, at the same time.
