Talking dirty

I love it when there's dirty talking during sex, it's one of the main things that makes me orgasm, but my SO doesn't really know how to talk dirty. He has tried before but I could tell that he was stuggiling coming up with things to say. Last night I asked him to try again and he flat out refused because he feels embarrassed he can't think of anything on the spot. I really want him to feel comfortable with it. any suggestions on what to do for him to get better/comfortable with it?