I would bring up to him that you don't feel comfortable talking about his ex first of all. You could even say something like you know it left him in pain and left a bad taste in his mouth and you'd rather see him not think of things like that...

Now for the waiting part it's truly just a question of how much you like him. A little background.. My wife and I were just great friends to start. We met freshman year in the dorms in college. The following summer, after a drunken party, we hooked up. We ended up being friends with benefits for a long time. In fact, it was probably around a year. It was the same way you describe it. Finally, after being inseparable for that long, I asked her to be my girlfriend. She knew I had wanted that for a long time but she had been hurt before me. I told her I would wait as long as she needed because it was worth waiting for. We have been married for 5 years now.

What I'm trying to say is some people need more time to heal. Some people need more time to commit. If he's worth it, never give up. Just be open and honest with him.

I hope my rambling helps