My son wants COD:MW2, but he won't be getting it. Although we allow him to play games that are rated "M", we still object to some forms of violence, "adult" situations and really strong language (though given what he hears at school we'll probably give up on that soon) and we don't think he's ready for that one yet. He won't be getting Fallout 3 for the same reason.
(In a way we are lucky because my husband and I are both gamers, so we don't have to rely on the ratings when we decide what our son can play. We play many of the ones he wants to play, so it's easy to decide if we're comfortable with him playing them. In the case of games we don't buy for ourselves, we make sure to read reviews or talk to our gamer friends to find out why a game is rated the way it is before we decide whether to buy it for him.)
He plays Halo 3 and ODST more than anything else right now. He will be getting Forza 3 for Christmas.
OP, I also suggest asking his parents if "M" rated games are okay, and also finding out if he plays online. If he has online access, you could get him Microsoft points for the XBox Marketplace instead of an actual game. He can use those to download games or lots of extra content for some of the games he has. My son will getting those cards for Christmas - he loves getting new maps for the Halo games, songs for Rock Band, or other DLC.