Boku no Papa ni Natte Kudasai

30-year-old systems engineer Haruki is overworked, clumsy, and a person who is hard on himself and others. One day, he meets the kindly Katsumi, a 52-year-old gentleman who takes care of him when he was about to pass out. Haruki ends up asking him, "Please be my papa!" and surprisingly, Katsumi agrees! As a pseudo "father-son," the two go to the zoo, a fireworks display, and experience things Haruki missed out in his childhood.

(Source: Houbunsha, translated)