It's not nice. In any context. I don't like jokes about rape on tv shows or when comedians make jokes. I don't like it in games. Unfortunately all anyone can do is be very careful about their child's Internet settings and view game/movie ratings before hand.
(Not saying that you haven't checked any of that before your son played it!)
Where did your son acquire it from? Presumably he downloaded it directly from the game developers, either paying a dollar or torrenting it.
It hasn't been through any vetting, because it was created by games developers and is only available directly from them. Infact, it has been rejected from Steam (a big online game retailer) for that reason.
I'd be more concerned about what else your son is accessing from the kind of sites that appear to sell this, but the "joke" is terrible, and probably warrants further conversation on why it's not funny or clever, as well as tighter parental controls.
I think you need to sit with him and ask why he was playing it and how he knew about it. If he's got hold of one horrible obscure title he'll be able to get hold of potentially many more possibly from the same place he got this one (was thinking maybe he's on a forum or a subreddit).