I feel like this right here may be the turning point that actually divides the Filipino people against Duterte. The exact quote below makes it seem like he would take his son's side in this.
In speeches, he had encouraged his son to attend the hearing but advised him to invoke his "right of silence."
Now Duterte has to choose a side. Stick by his word of cracking down on drug dealers and let his son be thrown and/or killed/executed or side with his son and show the people who have been following him blindly that his family is exempt from this law. If Duterte sides with his son, gets him off the hook and then his son ends up assassinated - which, I assume would be by a vengeful person who lost a family member to the killings - I would not put it past Duterte to enact some new law, targeting a new group of people, in spite of his son's death.
I say this is the turning point of dividing people up against Duterte because his decision rests on it. He sides with his son, the people who have been killing now see that the family of Duterte are free to do what seems to have been labeled the most evil and taboo thing to do in the country, which is drug use, selling or suspected of either/or. Some would even question why they killed a certain person, even if they ended up killing a friend or family or at the very least informed someone about the suspicion and got said person killed. On the other hand, if he decides to let his son go through with what Duterte promised, and his son is in jail or killed for what he did, then the people who were kind of in the middle between of the killings, could very well start to see that if he is willing to let his own son die or kill his own son, then what is stopping him from going further with killings.
All in all, I guess we'll see what happens with his son.