I'm going to take a different path here and play the DEVIL'S ADVOCATE by giving the JW view on this subject rather than the enlightened, common-sense, humane point of view that most of us have.
From the JW point of view, the parent's actions were completely justified - in fact they didn't go as far as the WTB&TS would have encouraged - they didn't beat him with a rod.
The parents are the authorities in their home, with authorization from god to maintain discipline and obediance. As an elder, the boy's father must demonstrate that he can administer the same sort of dispassionate justice as the god he serves, lest he loose his position as a loving sheperd in the Congregation of God.
This kid is not an impulsive child - he understands the rules, yet, like Adam, he chose to break them. He needs to be punished, and punished in a manner that will make a long-lasting impression on him. As humans, we remember those events that carry an emotional impact.
If the boy is truly repentant, he should ask his father to please stone him to death so that he might be assured a ressurection in Paradise.
His disobedience cost him his long-awaited graduation party. God is love. Hopefully this loving discipline from his paretns will preserve his life when Armageddon arrives. It is now almost as close as it was in 1975.