Help me learn how to communicate better

I'm in a sexual relationship with one of my friends. It's all physical, there's no love in it anywhere, which is fine with me. The only thing I don't like is that he doesn't seem to know how to make me cum.

He'll ask if I did, and I'll say no, and we'll just... move on. He's never gotten me there. I've gotten close, but he's always done before I am. I know I'm difficult to get to that point, and he knows that I was previously in an abusive relationship years ago with a guy who didn't care if I enjoyed anything sexual we did (which is why I can't easily bring myself to talk to him about this, because of what my ex hammered into my head), so I think we both tread a little lightly when it comes to the communication bit of it all.

I don't know how to talk to him to teach him how to make me orgasm. And I don't know how to show him what to do. Can someone give me some tips?