Ok. First of all, Common Sense knows nothing, NOTHING about games. COD 3 has NO blood, NO language, and is rated T. Not 16+. T means 13+.
Also, some games, like Call of Duty, World at War, have mature content filters. If you are dumb, then let me tell you that it means it takes away the blood squirts, the language, and the gore. Call of Duty, Black ops has that same filter too, if your kid wants it.
But don't listen to the actual Common Sense advice. They are those over proper, conservative old ladies who have the British accent and drink tea by sipping it and raising their pinkey. If you have any common sense, then you should listen to the kids, their posts, the adult posts, and most importantly, your child.
Sit down and talk with him. If you know him to to be an honest kid, then believe him when he says he's mature. Because he probably is.
But you should trust your own child not to go and kill people. It is just a game, and don't take this the wrong way, but when killing computer characters, you have a totally different mind frame than an actual killer or soldier. You don't take time to relish the blood spill, or think about what you did. You just score a point, do it again, then go to school the next day and brag about how you kicked your friends butt! It is just recreational!
So go ahead and buy the game! You don't have to buy a super gory cursing game, but don't be afraid to buy a mild M game. So trust your kid, and don't let him wonder in thought about how unfair this is. He'll thank you in a LOT if you let him just play the game he want's.