Ninkyou no Otoko ni Kawareteimasu.

Yukihiro, a college student with a peculiar constitution that causes him to become aroused by inhaling cigarette smoke, has spent his life avoiding people and struggling to make friends. His solitary existence takes a turn when he meets Senjo Natsu, a man who changes everything for him. As Yukihiro becomes “kept” by Natsu, he discovers a sense of comfort and safety he has never experienced before...

(Source: Kadokawa, translated)

Note: Chapter count includes 4 extras.