Is it permissible for a son to see his father's nakedness (only the thigh), even if the son is an adult, sane, and mature?

It is not permissible for a son to look at his father's thigh, because the awrah (private parts) of a man, according to the majority of scholars, is what is between the navel and the knee. They cited prophetic hadiths such as: "What is between the navel and the knee is awrah" and "Cover your thigh, for the thigh is awrah." Imam Ahmad, in a second narration, and Ibn Abi Dhi'b and Dawud disagreed with this, saying that the awrah consists only of the two private parts, citing the hadith of Anas about the Prophet, peace and blessings be upon him, uncovering his thigh on the day of Khaybar. The majority view that the hadith of Anas is to be understood as the loincloth uncovering itself unintentionally.