Itazura Neko Ni Goyoujin Manga

いたずら猫にご用心 ; Beware of the Mischievous Cat ; Please Note the Cat that Plays a Trick

Toshiya takes in his former teacher's unruly son, Tetsu, as a favor after Tetsu gets into some trouble with the mafia. Tetsu does everything he can to get Toshiya's attention, including having sex in his room, bringing home random guys, and molesting Toshiya every chance he gets. Meanwhile, Toshiya is gradually seduced by the store manager at work. The store manager and Tetsu fight it out through violence and more passive-aggressive techniques in hopes of getting Toshiya's affection---or his body, whatever.