@Frostisss: Indeed, that is the correct course for him to take.
The act of changing the past for whatever purpose is likely to be akin to using a steamroller to do a mallet's work on a nail and plank of wood. Even his changing his relationship with Haruka might result in unintended consequences reaching well beyond the circumstances of his own life (though it seems like whatever force brought him there functions to limit the number and range of such consequences--the story isn't about something so complex after all); imagine if he were to "change the world" as is so idealistically suggested by many facing the idea of time travel, and effect widespread negative change as a result of the multiple well-meaning changes he'd make. After all, "good" is often subjective, and dependent on attendant circumstances.
It'd just be the same tyranny of good intentions imposed by those with too much idealism and emotion and not enough logic.
Better he get his girl, make money, and let the world fix itself.