Inu mo Arukeba Koi ga Hajimaru@Hatsujouki

Hometani Sora, a plain high school student, unexpectedly begins dating his crush, Matsuda Makito, thanks to his unique ability to transform into a Pomeranian at a moment's notice. As they navigate their newfound relationship, the first mating season arrives. When Sora falls ill and misses school, Makito grows worried and decides to visit him at home...

(Source: Ichijinsha, translated)

Side story of Inu mo Arukeba Koi ga Hajimaru