Ano Hi no Kimi wo Dakishimeta nara

From Bliss:

Ever since Hidetoshi told his best friend Kengo that he was gay at graduation, and Kengo responded with “we’ll always be friends no matter what,” Hidetoshi was determined to keep things that way. Though Kengo realized that Hidetoshi probably wanted to confess to him, he keeps his mouth shut in fear of ruining their relationship. But when Hidetoshi gets involved with an abusive boyfriend, Kengo feels that he can no longer just sit back and watch…

A spin-off of the series Ashita no Kimi wa Koko ni Inai, which involved Mio-san's brother.