I've honestly never understood how a video game can be unsafe, as long as the child knows and understands that it's just a game and not acceptable behavior in the real world. Ratings mean nothing to me, and I've always let my kids watch, play, read or listen to whatever they wanted (except for porn, obviously). I've never had a problem with that.
My youngest son (17) has all 4 Halo's. They just came out with the 4th one, and I think it was pretty expensive. He has friends who are 12 and 13 who come over in the group of boys who hang out with him who play it. That group has kids from 12 to 24 in it. They all play all the games and stuff.
Unless a game has something in it that is very much against your religious or moral beliefs, I wouldn't worry about it. The biggest problem with games is that they can tend to spend way too much time on them and not do anything else, like their chores. It also leads in some games and such to wanting more money to buy xbox live cards and buy stuff for the game and all that. It can get expensive, hence the chores and outside jobs.
My advice would be to see if he can borrow a copy and play it and see if he likes it, and if he does then let him earn the money to buy it. World of Warcraft is one that will actually send you a free trial disk in the mail and let you try it for a month and then buy it, which is a good option especially if you have a game fickle child.
Pandora