Taiikukan Baby

Shibahara Jun is a high school student with high expectations for swimming, but as the team coach's son he is nicknamed 'Taiikukan Baby' (Gymnasium Baby) and no one believes his position is due to his own merits. Jun also must deal with constant rumours that his father got his late mother pregnant—who was one of his students—and never married her, another reason why he is so nicknamed. After being diagnosed with hypertrophic cardiomyopathy, Jun is forced to quit swimming and loses all reason to exist. Shortly after, Murai Naoki, his replacement in the swimming team, asks Jun to be his coach since he admires him a lot. Jun and Murai start to spend time together, and Murai soon confesses his love for Jun and kisses him, a fact that leaves the latter confused. Jun tells what happened to his best friend, Kato Shoichi, who takes an overprotective attitude towards him and another defensive towards Murai, since he is also in love with Jun. Both boys swear a mutual rivalry that starts a complicated love triangle.
(Source: Wikipedia)
~~ Adapted from the novel awarded the first prize at "The First Junon Love Novel Grand Prix".