Thank you, Jackson

Jackson the donkey works very hard, carrying loads of mealies, carrots and potatoes up the hill to the market every week, from the little farm where he lives with the farmer, his wife Beauty and his son Goodwill. But one day Jackson just stops halfway up the hill. The farmer pushes him, pulls him, shouts at him. But Jackson WILL NOT BUDGE.
Just as the farmer is about to beat his donkey, little Goodwill comes running up the hill. Quickly he whispers something in the donkey’s ear – and straightaway the old donkey gets to his feet. The farmer is astonished. What could be the secret of the boy’s message?