The Box that was filled with Rocks - I don't want a BOX with Rocks

A King who is worried about his son, a child that has become listless and unhappy with learning. The King sends a challenge out for someone to come up with an idea to put the enthusiasm back into his son. Someone comes up with learning and prizes in a box. His son loves the idea but when he goes to present it to his son an evildoer, puts rocks in the box.