Inu mo Arukeba Koi ga Hajimaru

Sora Pometani, a sober high school student, has a peculiar constitution: when he's anxious he turns into a Pomeranian. One day, his classmate and secret love, Makito Matsuda, bumps into him in his Pomeranian and ends up taking him home. Makito takes Sora in his Pomerarian form home because he resembles his grandmother beloved but passed dog, "Pometa." Makito finds out about Sora's true constitution and, in order to make his grandmother happy, he asks Sora to keep visiting her in his Pomerarian form. Sora is promoted from a mere classmate with a secret crush to one of Makito's friends.

(Source: Ichijinsha, translated)