Right, SO BACK ON TOPIC:
He's definitely mature enough, and lets face it people, I'm not talking about something like Gears of War or GTAIV, I'm talking about HALO. Come on people, this game was originally going to be T (until ESRB found out that the Marines swear a bit if you hang around them for too long just prior to the release of the Original Halo). And seriously, FLood Blood doesn't count for anything. Its like looking at a pile of vomit, which obviously isnt bad unless youre queesy. You have to really look for the blood in this game to find it. The only really bad thing is violence, which really is only a problem if you are 10 or below. He's not a very serious gamer either, He just beats the single-player and plays a couple hours of XBOX-Live every week.
And for COD, its the exact same thing, especially when you turn off blood and gore effects. Thats what I do too, and it just makes it like a T Game.
My final arguement against you nay-sayers is that it isnt photo-realistic like a movie. The fact that your actions are limited and there are minor graphical issues here and there shows that its just pixels. You'd have to be really immature to get scared or believe its real (except in the case of F.E.A.R. or Bioshock).
I'm not even kidding, he's probably smarter than almost all of you when you were at his age. So yes, he is potty-trained, can scratch his armpit better than a newborn chimp, etc. 
And I also need an arguement stating that shooters are not pointless and stupid. Or an arguement saying, So What?
Yes, shooters are not the most cerebral of genres out there, but neither are MMOs, some RPGs, some sports games, and possibly CnC (considering you can just win the game through a tank rush). But still peole, its supposed to be FUN, and thats what recreation is about. Plus, if you tax your brain too much then it does have a habit of degenerating.
And for those of you who doubts this kid's intelligence and responsibility, I'll give you an example. We do a thing called Science in THe NEws for our Advanced Science Class. You basically type up a 2 page report on some recent scientific event. You are given four weeks. He does this on the first or second week it is given, as do I for that matter. And he consistently gets an A on this assignment. Additionally, he has an A in all of his classes, so I should think he would be fine.
As far as the social aspect goes, hes fine. He is a good swimmer and loves sports. He has got plenty of friends, definitely a fair bit more than me, since I spent a ton of time reading and learning, so I have a habit of discussing things people dont care about like Global Warming, Climate Change, Political Corruption, Etc. (which really bothers me, considering its supposed to be a gifted class, but then again I guess these people want to have fun in the short-term and haven't realized that they can make a difference even at this age).
So thats about it, keep at it people!