Does one who withholds the staff truly hate his son?
Rashi: In the end he will hate him, for he will see his son act evilly.
Malbim: If he withholds the staff due to affection for his son, he hates him, for via this, his son will not learn Chachmah - "Ben Chacham Musar Av" (1). This is a sign that his yearning for his son, which does not let him see his pain, overrides [concern for] his son's benefit. He loves himself more than he loves his son.
What is the meaning of "Shicharo Musar"?
Rashi: He constantly chastises him each morning. Malbim - also this answers why Hashem afflicts Tzadikim, and "v'Yesh Nispeh b'Lo Mishpat" (23) - due to His love for the Tzadik - "Ki Ka'asher Yeyaser Ish Es Bno Hashem Elokecha Meyasreka" (Devarim 8:5). He afflicts him with punishments, for Musar and to improve him in his end.
