Ichinashi Kimi Manga

From Exiled Rebels Scanlations: Toujou Aki lives with his younger step-brother, Saku who is a university student. Instead of going home to sleep though, Saku will stay out with as many women as possible, to the point of being a man-whore. Aki, on the other hand, has a deep hatred for women, enough to make him nauseous being around them. Aki's one secret is that he would do anything for his brother. No matter what it is. However, everything can change in an instant during the silence of three minutes.

A high school boy who's always considered himself to be an alpha finds out that he's an omega?? And his destined partner is a kid?