Telling a Father That He Committed a Sin

Hello and blessings 

It is forbidden for a son to say to his father, “You transgressed the words of the Torah.” 

It is permissible to tell stories in the father’s presence about people who hurt others and were punished, or about people who asked forgiveness and were helped. 

If the father asks on his own, “Is there something I need to correct?” it is permissible to answer him in a respectful manner: “Perhaps so-and-so was once hurt by you.” [They should not say, “You hurt so-and-so,” or, “You behaved toward him improperly.”] 

Certainly, the sons should try to help as much as they can to bring him out of his difficulty, with assistance, encouragement, and the like. But they should not “help” him by conducting his “soul-searching” for him...

Shulchan Aruch, Yoreh De’ah, siman 240, se’if 11
If one saw his father transgress the words of the Torah, he should not say to him: “You transgressed the words of the Torah.” Rather, he should say to him: “Father, it is written in the Torah such-and-such,” as though he is asking him, and not warning him; and he will understand on his own and will not be embarrassed.