The Orphan Master's Son is Jun Do, an orphan who creates a family story starring the cruel head of his orphanage, Long Tomorrows, and the beautiful singer whose picture is pinned on the wall. Jun Do has no intention of accepting his fate as an orphan: orphanhood is a pretty bleak prospect in impoverished and oppressive North Korea.
By adopting the Orphan Master and his faithless lover as parents, Jun Do convinces himself that he won't share the fate of the other orphans. Even though the Orphan Master clearly has no love him, Jun Do senses that he will need something to distinguish himself in order to survive in his society. So when he is pressed into military service at a young age and given truly rotten assignments—just like all other orphans who survive past childhood—his continued denial of his true identity insulates him from the soul-crushing reality that no one cares about him.
There's also a subtle reference to the other Orphan Master in the book—Kim Jong Il. We're told a tale about an orphan and a bear that adopts him and feeds him honey from his claws. It's an animal allegory, one in which the Dear Leader (the bear) cares for all the fatherless in North Korea. If Jun Do fills the role of the orphan in this story, the conflict between the two is more than just the conflict between dictator and oppressed citizen: it's a family feud. And Big Daddy has no problem crushing his upstart son.