Yoake no Little Mermaid

Makoto loves singing but after a certain incident, he doesn't sing in front of others anymore and now he only does so while facing the sea. Then one night, he meets a young boy called Nobuaki. He found himself feeling comfortable with Nobuaki just quietly listening to him singing but by the time he realised it, Nobuaki had disappeared. Makoto has found himself unable to forget about him ever since.

This is the slow and long story of unrequited love between an unemotional guy and a commendably single-minded guy.

(Source: Homesha, translated)