Honestly, this strikes me as one of Rockstar’s more mild games compared to the Grand Theft Auto series. It plays a lot like Grand Theft Equine, but what I’ve played through so far is not much worse than an old cowboy movie. Obviously there’s profanity and violence, but the main character himself is supposed to be a “family man”. There’s the opportunity for Rockstar rampages, but they’re not nearly as brutal as some of the massacres in GTA IV.
Of the Rockstar games, Red Dead Redemption is the one I’d feel safest giving to a 13 year old, but considering Rockstar’s game repertoire that isn’t saying much. I personally wouldn’t see a problem with it, it would heavily depend on the kid and how mature he is.
And please, for my sake, keep him off a xbox live mic if he hasn’t hit puberty yet.