It already said "Ben Chacham Yesamach Av u'Ven Kesil Tugas Imo" (10:1)!
Malbim: There it discusses via himself. His father rejoices over his Chachmah, and his folly grieves his mother. Here it discusses when he matures. A Ben Chacham inherits the laws of Chachmah and goes in them. He strives to gladden his father with his good deeds. Also this is from the laws of Chachmah, to gladden his parents!
Why is he called "Kesil Adam"?
R. Yonah: Also to Stam people he is called Kesil, for also commoners honor father and mother. Adam is the Klal - "Nefesh Adam" (Vayikra 24:17). Often it refers to commoners who lean towards desire and their deeds are earthly, and not of the Nefesh of Chachmah. "Ach Hevel Bnei Adam Kezev Bnei Ish b'Moznayim La'alos Hemah me'Hevel Yachad" (Tehilim 62:10) - their weight (importance) is not considered anything. Bnei Ish are people honored in the world for their Sechel and diligence. "U'Chsil Adam Yevale'enu" (below, 21:20) - one who consumes his father's storehouse, also commoners call him a Kesil.
Why does it say that a Kesil disgraces his mother?
Rashi: He causes that others disgrace his mother.
R. Yonah: A Kesilim disgraces anyone whom he does not fear, even if he is obligated the ultimate fear and honor. Da'as does not force a Kesil, only a staff and fear. Therefore he disgraces his mother, for her staff and fear is not on him like that of his father.
Malbim: A Kesil is drawn after his desires. He disgraces his mother in action. Also this is from folly, to disgrace her for rebuking him about his ways!
