Until the Full Moon Vol 1

From the Kodansha edition

Marlo has a problem. On the night of the full moon, this half-werewolf, half-vampire undergoes a mysterious and terrifying transformation: he turns into a girl! Desperate for a cure, his parents call on Doctor Vincent, a long-time family friend. Marlo objects, wanting to keep his condition secret from Vincent's son, the vampire playboy David. Unfortunately, the secret gets out, and a new problem surfaces: David is interested in Marlo's female form! If a remedy can't be found, their parents believe the next-best solution is marriage - a marriage between Marlo and David!

Source: Anime News Network Encyclopedia