My guess is that it was that boy who’s on his father’s side and is going to be the nex...

I also guess that can be him, but at the same time I think that it would be very poorly written. First because it would be too convenient he appearing and at the same time someone wanting to kill him. Second thing is that he is very close to leewon father, as a adopted son. He has that trust of him and wants to protect him and he probably know that kill Leewon could be fatal for his health.
So I hope not. But I wouldn't be surprised