Language is PG-13 as is violence. Its not like Gears of War where you can curb stomp people. You are killing aliens with blue blood and zombie like creatures with green blood, so I wouldn't call it gory. If you are a little sadistic, you can kill the marines that accompany you, which is M rated behavior. In online play you shoot other people, but they are heavily armored, so again, no gore.

If he is going to play online with chat on, then the language will be R-rated.

I recommend you watch some gameplay clips on YouTube and make up your mind that way.