Seija Musou: Salaryman, Isekai de Ikinokoru Tame ni Ayumu Michi

A salaryman collapses from a gunshot wound. When he comes to, he finds himself reincarnated in another world as a fifteen-year-old healer named Luciel with a talent for holy magic. In order to survive in this new world, he must build up his body and endure intensive and strict physical training, not to mention awful flavors!

(Source: Kodansha USA)