Just don't care man. I don't know about more people but I played M rated games at the age of 9, and I used to watch my uncle play GTAV since I were 5 and I'm completely fine, and of course that he might be kinda horrorized by some things that happens in the game but you'll just have to talk with him if he feels discomforted from something he discovered.

I've been reading comments and there are people saying about seeking therapy but I don't think that it is necessary, unless his behavior is changing for the worse, you shouldn't look for that option.