Well, it really matters what kind of gamer he is...
Hardcore Gamer - Plays frequently and usually likes action-packed games like fighters and shooters.
Part-time Gamer - Plays casually and likes fun games anyone could like, like party and platformer games.
Hidden Gems - Plays pretty much everything and likes to find those games behind the scene that nobody really notices.
Sports Player - Plays games based on a wide-arranged category of sports.
Family - Likes to play games that the family or friends like to play, usually multiplayer or family friendly games.
Based on these categories here are the games I'd recommend
Hardcore: Resistance 2 or Fallout 3
Part-time: Guitar Hero: World Tour or LittleBigPlanet
Hidden Gems: Valkyria Chronicles
Sports Player: NHL 09 or MLB 08: The Show
Family: SingStar
Good luck, I hope I've helped!