Uraura Hikaru: Shinshin ni Mitsu

Keisuke spends his days as if he has died after breaking up with his beloved girlfriend Chizuko. One day, he meets a boy, Koushi, who claims to be the child of Chizuko and Keisuke. Keisuke is happy to have a connection with her, but is told that Chizuko is dead. Koushi, who was brought up by his grandmother, wants to meet the recipient of her transplanted organs. Why couldn't the two of them be happy?

(Source: Mag Garden, translated)