So what are you going to do? You cant keep the game another 5 years..it will be well out of date by then? Id advise taking it into GAME etc to part exchange it..but as you say your son has spent a bit on the online maps etc. Downloaded via the Xbox marketplace.
My younger brother is 14 and he has this game. Hes only allowed to play it every now and then, and only online against his friends.. Hes trusted by his parents, and knows if he abuses that trust the game goes.
If your son has that game, its likely he has other games that are age specific.
And not wanting to tell you how to be a parent, but this is a game his Dad has bought him. This may be a case of Dad being hey - Im cool??
The pair of you should know what age relevant items he has, esp as he gets older hes going to come into contact with more 18+ items (videos, magazines etc), and set ground rules.
And imo no its not that bad. Its a game where you shoot people.Yes its violent, but unless your son is a psychopath in the making, its not going to affect him adversely