just show him this video :)
http://www.youtube.com/watch?v=eE_msAKWdOs

Teach him that Call of Duty is a clichéd unsophisticated mammoth stealing away sales from other great games that deserve to be recognized beacuse they fullfill the needs of a gamer better in the long run.

Tell him that as a true gamer he needs to ignore All of Duty and support the competition and niche games.

all you have to do is take away his console.....that's what my mother did when I was young and my grades were slacking.

If youre serious with this and he's really an addict, you might have a problem. (Damn, i remember when i was WoW addicted for 3 like years.. )

This may sound awful, but the best idea would be to let him play the game for some time, and then destroy his savefiles and his XBL (or PSN) account somehow., he'd be so discouraged that i think he'd lose quite a lot of motivation. (or flag him for cheating on said services, leading to him beeing banned)

In the long run, he needs to A,) get bored of the franchise, or B.) find something thats far more interesting to him than CoD.

Depending on what kind of guy your son is, a therapist would be an option, but this only works if he knows about his situation, and does care in the slightest. Another huge problem would be that his friends (so i guess) are more or less the same (only if they actually are, of course), meaning that they'd hype each other to death on that Franchise. And unless they change too, the task to change your son's behaviour might be next to impossible.

Didn't read the whole thread so sorry if this is mentioned.
Perhaps a preemptive grounding that he can only play COD on weekends until he shows his grades maintain at the level you want than add 1 day a week of COD play as long as he keeps his grades up.

Someone his age being "addicted" to Call of Duty is not out of the ordinary.

In this, he is fortunate in having a father as informed as you. Teach him about other games. Show him Half-Life and BioShock if he wants shooters. Get him to play some innovative indie games and see there is more to gaming than just FPS.

And, of course, tell him that if his grades drop, then no CoD for him.

Moderate him

When will he learn that single player gaming is better than online gaming.

Just lay ground rules, such as only on weekends or limit gameplay to an hour a night. You have power over the internet connection!