I am fortunate enough to have a dad that doesn't care about mature content in video games. I played Doom with him when I was five and it never messed me up. Even then, we both understood that it was just a game, and if I went and pounded on my brother in real life, he would give me something else.
Also:
if he's 13 I can almost guarantee you that he's been exposed to stuff that's a lot more graphic and violent than Skyrim. I'm pretty sure most 13 years olds know quite a few swear words and are pretty much surrounded by adolescent swearing hormone induced teens at school.
I completely agree with this. One of the major consequences of being an over protective parent is that your kid
will develop ways to circumvent your attempts to control him and he
will begin discovering stuff like this on his own, and this could take a toll on his character.
Really, what you should do is
talk with your kid about violence/six/language, and then selectively indulge him in some media that makes use of these things. In other words,
let him play and watch stuff with questionable content in it, and maybe even enjoy it with him. If you take it away from him altogether and refuse to talk about it, he'll just want it more, and that will drive him to get it from other sources - and as I said, that's dangerous.