I don't think Hajin is their son, they had shown that Hajin had a certain accent from some place when he was a kid. So we can assume that Hajin is either pick up by the director to hide Hayeon existence since media caught the mother is visibly pregnant. So to protect both of their image, Hajin is being adopted.
Oh yeah, damn, right, she was seen pregnant. And that baby was Ha-yeon, who was conceived in an affair. Maybe the dad wanted a child who had some of his, like, characteristics, so no one would call the mom a cheater. And so they found Hajin, maybe he was from a poor family or already an orphan, and that's how they were able to adopt or buy him
Yeah. I was just assumed that since she was a rising star and famous celebrity, so her marriage to the director of some kind of rich company would definitely gained some attention. So her being pregnant might be caught by the media whether she likes it or not. Since the father never really talk anything about Hayeon existence either and just show that he is also violence towards him, well, he definitely not liking Hayeon either and it's only the mother that affectionate towards Hayeon. So yeah. The possibility that Hayeon is not the father's son is high.
Theory time! Ha-yeon is the kid the mom had either from a random affair or from the person she actually loves, and he looks like her and means more to her and that's why she wants to actually have him in her life, and Hajin is the kid she had to have with the man she was forced to marry (or Hajin is a random kid they found who looks enough like the mom and dad, because the mom refused to have sex with the dad or was unable to have kids). That's why Ha-yeon is all like "You stole from me"