Ore no Koigokoro ga Moteasobareteimasu

In order to drive away his persistent previous boyfriend, Yuzuha, one of a set of twins who are popular influencers, declares he’ll start a relationship with the passerby pudgy underclassman Yachiro. Although he was partially caught up in it, the kind Yachiro went along with it and agreed to pretend to be his boyfriend. Yuzuha fully enjoys playing at being lovers with warm and squishy Yachiro. However, when Yachiro loses weight and drastically changes into a gorgeous man, his heart won’t stop fluttering...!?

(Source: Gush, translated)