Basically, anything that is on your mind. You might start by saying "So, we've been dating for a while. I'm not saying it's going to happen soon, but I want to talk about sex before it's too late." And ask him whatever you want to know about his history, preferences, beliefs, etc. Don't get offended if he asks you some personal questions as well.
If you're looking for something a little more fun down the line, cosmo magazine's website is a fun tool. Discuss things on the site with him.
