Don't liten to all these people who keep saying "It will mess up your kids! It will make them killers!" I've played GTA since I was 10 years one and i'm fine. Allthough if it starts changing your child's attitude (cussing, becomming more voilent, etc) don't let him play it!! If you are/have been a good parent and have taught your son right from wrong and done your job as a parent then playing GTA would be fine for him. Just watch him play some times...does he seem like he's enjoying it a little "to much"? (If ya know what I mean)
Steven is very right. If he understands it's just a game and treats it as such it's ok...but if it affects him in ways a game shouldn't then he shouldn't play it.