During Act 1, try spamming partial rests (Full rest, no supplies) until you see him conjuring an image of Mystra.
Should let you have a conversation where he offers to help you channel the weave. There's literally an option that talks about how "Intimate" it is, which should set you on his romance path.
Or he might compare you to his cat. In which there's dialogue options where you say you'd hope to be closer.
So long as you end up resting enough to provoke a conversation (And don't outright reject him) he should be open to a relationship.
There's also the Elminster encounter which you can advocate your desire for him too.
