The main theme could be the struggle between ordinary human nature and the concept of a superhuman. In the story, the man might represent the common flaws and virtues of humanity, while the superman embodies an ideal or a set of extraordinary qualities that the man may strive for or be in contrast with.
One possible theme is self - discovery. The man might be on a journey to find out his true self, and the superman in the story could be either an obstacle or an inspiration. The contrast between the two characters can force the man to re - evaluate who he is and what he wants to become.
It might be about the exploration of power and responsibility. The superman typically has great power, and the story could be delving into how he uses it. The man in the story might serve as a foil to show the difference in how power is handled by someone normal versus someone superhuman.