I showed a 6 year old boy Monster Hunter. it's a violent game that's about well, hunting giant monsters. his Mom lets him play M rated games, so i thought it was ok. but was that a sin? I'm scared now because Jesus says that if you cause a child to stumble, it would be better for you to be strangled by a millstone and thrown into the sea.
That game would not be likely to cause him to stumble, it does have quite a bit of blood though and monsters that maybe scary for little kids.
But I would be more concerned if his mum lets him play the modern Call of Dutys (especially if she lets him play online multiplayer), Far Cry, Yakuza or GTA, most violent M rated games aren't just a bit of blood here and there as they used to be with not so realistic graphics, now a lot of M rated games have bad language in them and even some have sex and nudity in them, I remember the first 2 Call of Dutys I don't think I even heard an S word in the original (unmodded) games and blood wasn't as excessive and not as realistic as in most M rated games today, the sound effects were there though, and now too if he plays online multiplayer, voice chat is now a pretty big thing, and some players use a lot of bad language. I myself even try to do a little bit of research of what sort of content is in a game now before buying, even as an adult.
So just be careful what you show him.