Kono Jinruiiki no Zelphy

From MangaHelpers:

Reesha was once a prince of a powerful dynasty, but after the end of a war, his home is no more. In the the first three years of Reesha's new life, he went to school with some of his family's former subjects, now his friends. However, after they graduated, they all joined the military. Reesha's small size, due to his artificial heart, has prevented his acceptance into the military for three years running. Now, 19-year-old Reesha is living as an ordinary plumber in a town of hot springs. In order to help Reesha get around the size requirement, he and his military friends sneak into a bathhouse to appeal to the general directly. Unfortunately, this plan results in a series of unexpected, violent events which cause Reesha to finally depart his everyday life and head off to chase his destiny! [tethysdust]