Revival Blue 

It was the summer of 2009 when it all started for Minato. Fourteen years old and preparing for his high school entrance exams, he meets Naoki Okisaka, the long-term guest of a small guesthouse inn Minato's mother runs. Naoki, a Tokyo resident who once used to live in Minato's town, immediately intrigues the young Minato who dreams of someday going to Tokyo and exploring the world outside of his small hometown. Minato drags Naoki along on his shenanigans with his best friend Akiyoshi all summer. But eventually, the time comes for Naoki to go back to Tokyo and for everybody to return to their realities... but nobody really forgets the summer of 2009. Not even years after when Minato and Akiyoshi have grown up to be full-fledged adults. (Source: Futekiya) Note: Includes three extra chapters.