If you're looking for manga similar to Kimi to Boku (2001), you might like these titles.
The earth has entered another ice age after civilization was wiped out by widespread flooding, and the remaining survivors must be self-sufficient. Yuki, a worthless NEET, is living with a young girl named Rikka, who is of weak constitution and always losing weight. She reluctantly gets a job to make sure that Rikka can have three delicious meals every day.
Kentarô Ueno is a mangaka. He lives with his wife and her 10 years old daughter in a small house that is also his studio. They are a happy family. Even though his wife suffers from depression, she is following the treatment, and all seems fine. One day, he found her on the floor, faced down and inert. Without Saying Goodbye is the story of what happen after that in Kentaro's life.
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 etc so Haruki can have the experiences he missed out on during childhood.