I am going to give you the advice from a man who was sexually abused. He is probably feels shame about this, and your constant questioning of him is going to cause him to withdraw. If it were me, I would let him know how much you care about him, and let him know that when he feels safe enough to talk to you about it, that you are there. Everything has its own timing, and this is something you cant force, and trying to make him confront this before he is ready could have dire consequences.