I have recently seen two video clips of GTA - no one will play them in my house, I don't care how old you are. Fucking a prostitute, killing her, then retrieving the money off her body will not be 'normalised' through game play. End of.
No other first person shooting games either. I don't know a single person who has played them who isn't a bit of an arse afterwards. No thanks.
I would explain (as someone else said too) about growing neural pathways and the fact that I don't want his brain 'wired' that way. I'm not his friends Mum and so can't have any say in how they're developing. It's not that you think they'll starts shooting people or using/ killing or prostitutes, it's about the level of aggression and violence that becomes normalised.
It is also about the people he will come into contact with playing these games. I know adult gamers who have stopped playing or who will only play in closed groups because of how nasty it gets. One serious gaming MNer pretends to be a bloke due to the abuse she was getting when she was getting when she was open about being female.
If explaining to him why the answer is 'No' doesn't work then I'd say 'Find other games or find something else to do with your time. Any more whinging and the console is out of action for a week. Don't ask again because while you live here, the answer will not change and I will keep taking the console off of you'.
Being told no will make you on popular, but you lived through the toddler years, you'll live through this too, so will he.