How is a "Ben Chacham Musar Av"?
Rashi #1: The verse is abbreviated. A wise son asks, and loves Musar of his father.
Rashi #2: Due to his father's Musar, he is a Chacham.
R. Yonah: Ben Chacham is in his youth, like "Ben Hayisi l'Avi" (4:3). When you see a Ben Chacham, it is known that his father chastised him, and he heard his Musar.
Malbim: Ben Chacham is one who conducts according to the laws of Chachmah. They are difficult for the heart's nature to accept and conduct according to them. The images of desire are weeds that arise on the land of the Nefesh. One may cleanse the Nefesh via Musar. He afflicts his son and informs his of Hashem's punishments and the evil that will come on one who departs from the laws of Chachmah. While he is a youth, he afflicts him with a staff until the images of Chachmah rule in his heart. A Ben Chacham is a sign that his father's Musar affected him.
Did the Letz not hear rebuke?
Rashi: He does not accept rebuke.
R. Yonah: When you see a Letz, know that this occurred because he did not hear his father's rebuke - he never scolded him, and withheld from him the staff. We learn that a father must check his son in his youth, if he engages in frivolity, idle talk and Leshon ha'Ra, and afflict him, for "when he matures, he will not veer from the habits of his youth" (22:6).
Malbim: A son who never heard his father's rebuke, not only will he not be a Chacham - he will be a scoffer who mocks the laws of Chachmah.
