Idolmaster Dearly Stars: Neue Green

From Manga-Updates

Ryou has always had a problem with his feminine appearance. As a child he was often mistaken as a girl, and to his disappointment, that didn't change even after he got to high school. One day he came up with his best idea yet for gaining masculinity; he decided to become a male idol!

His cousin Ritsuko worked in the idol industry, so she was able to get him an audition. Unfortunately for his master plan, the company decided that he would make a wonderful female idol. Now, to even have a chance at becoming a male idol, he must first become successful as a girl!