This character is a transman.

Transsexual people experience a gender identity that is inconsistent or not culturally associated with their assigned sex, and desire to permanently transition to the gender with which they identify- usually seeking medical assistance to help them align their body with their identified sex or gender.

A trans man is a man who was assigned female at birth, but who has already made a permanent or partial transition, or have recently started to transition to the opposite sex (this could be before or during the story).