That's very sweet Elu
it could always be a choice made during a conversation near the end of the game. It could be something like "but lass, I hear the song. I must leaving for my Calling soon. Our time together is nearing its end." And the quizzy could choose between options like "I'll miss you" and "you're not going alone. I'm with you till the end." And some heartless break up line I don't even want to think >.<
Or...he may feel that he'd never forgive himself if you'd go with him to the deep roads.
on your last romantic interaction with him and he invites you for a drink (it's just a normal drink, perhaps not even the first time he invited you, just to make the player unaware of what he's about to do), he could spike it with a sleeping drug and go off himself to the deep roads.
maybe leaving you with his most heartfelt last letter/will to the inquisitor before the end.
kinda like how alistair goes off to kill the archdemon, sacrificing himself instead of the warden.
ahhh...feels good to crush hopes and dreams 
no wonder the devs are hooked on the stuff