That it's not funny is in first of all his oppinion and since he's the issue, that should be respected! He doesn't talk that way for fun, I assure you that. He talks that way, because it's the way he talks. That he's not talking like that for fun doesn't mean that you should be offended, because the intention of using words like "cunt" and "fuck" is in his case different than in yours.
You're not earning yourself much of respect here btw. by telling he hasn't deserved respect... this is bullshit! Giving respect to someone is your choice and nothing that the other needs to earn from you. And everyone has the right to be threatened with respect in a first.
If this makes you understand better, it's not about the words that he or you are using. It's about the fact, that this way of talking is his normal way of talking and therefore part of his identity. If you imitate that, you are making fun of him, not with him...