"a son": and not a man (one who has reached the state of manhood.) A minor is exempt, for he is not obligated in (the observance of) mitzvoth.
"sorer" ("who turns astray") "umoreh" ("and who teaches"): two times. He turns astray from the Torah, and he teaches himself a "different way" (viz. Ibid. 20) "he gluts and he guzzles."
(Variantly: Just because he used his father's money he must die?! Only he is judged based on his future behavior - better he die while [relatively] innocent and not for more severe sins. His father desired a captive woman and brought the devil into his home and caused his son to be rebellious. The result will be that he will kill him in an unnatural way, as it states (v. 22) "And if there be in a man a sin whose judgment is death, then he shall be put to death...").