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.
I'm a Foreigner, and as such, i am grateful for everyone pointing out any mistakes in my english posted above - only this way i'll be able to improve. thank you!
