Zip Up Boy

Zip Up Boy summary: High-school student Kazuki begins a casual sexual relationship with an older man after meeting in a bar. He becomes addicted to the affection but is perplexed by the indifference of his partner. Meanwhile, Kazuki's mother has decided to remarry Akira, the nicest guy you could imagine... unfortunately his brother Reiji just happens to be Kazuki's cold-hearted playmate.