He usually opens up to you most strongly if you help people, especially children, in need. Protecting Sazza, Arabella or Mirkon (and I think a few other things) trigger small conversations with him in which he talks about his past. When, during the scene with the image of Mystra that has already been mentionend, you reach the portion of the spell where you are supposed to picture an interaction with him, pick one of the two obviously romantic ones. From what I have heard (I have either romanced him or played as him, so my experience of just being friends with him is none-existent), you close his romance route if you pick one of the friendly or hostile actions here. But others might disagree.
