[Spoiler]
He leaves on every route, it's part of the plot. I'm pretty sure we won't go too long without him seeing as this is a romance, and we'll probably have tutoring-related timeskips soon. By trial and error, these are the choices that raise romance once you're on his route:
CH6: State that you don't think he's as selfish as he makes himself seem.
CH7: "What about you, Xelef?" You expect an apology from him as well.; then: Let it go.
CH9: Let him go.
If you've gotten all of his romance points in the common route, I am fairly certain this will get you to the maximum amount of romance points he has available at the moment. Have fun.