Lost Boys (2004)

Blurb:
A boy named Air suddenly appears at Mizuki’s window one evening, forcibly transporting him to Neverland. Having grown up most of his life without his own father, Mizuki wants nothing to do with surrogate fatherhood. Unfortunately, stuck without a way out of Neverland, he has little choice but to stay. As he spends time with Air, however, Mizuki is gradually charmed by his boyish innocence, and the two fall in love. In the end, Mizuki decides to return to the real world to reunite with his estranged father, but tells Air that he will always leave the window open for him.

Source: www.anisearch.com/manga/14