I don't think you're being unreasonable, per say. I just don't know that you are doing what will make your son learn a lesson. Instead he is going to rebel and hang on to this kid for dear life.
You're at the age where your SON is going to have to pick and choose his friends and if you "get in the way" he won't learn how to do it. So, don't block him.... guide him.
I'm not sure he's in emminent danger by being at his friends.... so let's have perspective.... although I do understand your complete frustration. He's not going to get hurt.... you just see him going down the wrong path if he's exposed to these things, right? Well, he's going to be exposed for the rest of his life because there will ALWAYS be another friend like that boy... .and they are the fun ones!!!!! So, next time, he'll get more creative and not tell you what goes on because he doesn't want you to pick his friends. So, now he's lying to you by omission. Then.... when he can drive... he starts to tell you he's going to one friends house, but goes to another's.... now he's outright lying.
Instead..... TEACH your son that this is not the kind of boy who is BFF material. So that YOUR SON can decide he'd rather have friends that have more in line with his own value system.
specifically about the games.
Can you talk to your son about what his thoughts are about playing those games? I mean REALLY talk to him, because his first answer will be 'it's cool' But if you ask him questions like "what do you think happens if you really shoot someone?" "how do you think those girls feel about themselves, dressed up like that?" "What do you think happens to your body when it's all amped up like you get when you play those games.... how do you get yourself calm and NOT want to shoot something?" "how does that game teach you something that you can do in real life"
About Facebook
Ask him why he thinks its a rule that you aren't allowed to have a facebook account until you're 13. Then show him stories about how kids are bullied on facebook and ask him how it feels. Give him info on predators and how they are on facebook and how people are not always who they seem to be on facebook and let HIM come to the conclusion that he shouldn't be on facebook.
Then - ask him if it's hard to say NO to a friend when they want to do something that you know is not allowed. This is a skill he ABSOLUTELY must learn soon. peer pressure set in about 2 years ago and will ramp up bad in a couple years. So, he needs to be able to say- "I know you are allowed, I don't want to do that... let's do something else". Because today it's about an M rated game, but when he's 15 it will be about alcohol or drugs or sex.
By you 'banning' him going to the friend's house you haven't done him any favors. You've just made him resentful of the fact that he thinks you guys suck and he misses the fun. Instead..... guide him to where he gets that the fun isn't always what it's about and why fun can be bad for you.
Just my $0.02
B.