I won’t say the ML is clean but as this so called revelation is made by the shitty grandfather, I wil, take it with great precaution.
For me the ML (Do-yoon) has feelings for the MC (Yejun) till the beginning but because of the shitty grandfather he has some kinda of trauma and decided to break up with him. I think he has some mental issue like he imprisoned his deep self. But now that he know he has a son who can’t be hiden from the shitty grandfather because of the illness. So I think he’s playing a double game with the shitty grandfather.
Many people say Do-yoon just have to talk with Yejun but it’s not simple. He is inapt to do this because till his childhood he is told bad things by the shitty grandfather, he saw his mother unhappy with him and saw himself like a monster. So yeah he need to do something about this lack of empathy but no basically as he is now, he can’t just sit on the sofa with a cup of green tea. He has to face an even worse monster, the shitty grandfather who can go to hell !