You Own That Info
In my opinion, you are under no obligation to tell him that you were raped. You own that information and it's entirely up to you whether or not you want to share it.
When you start getting close to someone, you want to share intimate details about yourself with him/her. You want that person to know your joys and especially your fears and your pain.
Tell Him When You Trust Him
But this takes a certain level of trust. When you get to that point, you will know. If it's important for you to tell him, do it then.