I feel like the ending can be 2 things, bittersweet or happy.
Bittersweet: He goes on to graduate high school and finds a nice girl and has a few kids, the whole time never forgetting Cauis and regretting his choice.
Happy: He realizes right after what a mistake he has made a somehow comes back to Cauis.
Whyd the author have to add that aweful part about him not living very long? It makes the ending so bittersweet....its like reading a John Green book :/