I have a friend from college who I think may have mild autism. He is extremely intelligent knowing all manner of world affairs, trivia, etc. and generally witty and fun to be around. However, he says things that are wildly inappropriate at times that are often denigrating to me and really upsetting. For example, sometimes he might make a joke about the holocaust that is incredibly rude in polite society but is hilarious in context. When he takes it too far however it is enraging. He can say things like "Oh [me] said it was OK later if we take [my girlfriend, not present] out back and roll train (gangbang) her." He does not seem to say this about other people and genuinely believes he does nothing wrong when saying them. When confronted about his actions he says things like "I didn't say that!" or "What did I do?" Or things of that nature.
The line between appropriate and inappropriate behavior is hard to draw sometimes because of the sense of humor that our group of friends has but clearly there are circumstances that are upsetting to me and I've made it clear about it before. We're both men in our late 20's.
How can I confront my friend about his inappropriate jokes without being immidiately dismissed?