They usually say that the apple doesn't roll far from the tree. So the father's accusations automatically fall on the son, who essentially decided to clear his father's name - otherwise, why would the son take his grandmother's surname, if not confirmation that "the sins of the father fall on the son"?
"the son is not responsible for his father" - he is quite responsible, ESPECIALLY if the son decided to accept his father's inheritance.
and yes, if anything, I would advise studying your inheritance law. It probably says there that the one who accepted the inheritance is obligated to bear responsibility for the debts of the one who left the inheritance. and in our case, the debt that the father left is an accusation of rape. and this debt must be paid, including by clearing the good name of the father, who was actually innocent, that is, the son must prove the innocence of the father.