This is a very nice story. I think the old wise man did really great in giving this young boy a method of which he can relate to.
First of all, he chose a method that was highly likely for the young boy to carry out without fail. The wise man also made the boy think... after all, we don't grow up with just knowledge, wisdom is the key to growing up.
What I like most was when the wise man asked the boy about the holes that was left behind on the wood. I think here, the wise man tried to tell the boy 2 things... First, is that whatever damage he has done with his anger initially, there would be scars left behind no matter how we try to fix it... Second, the karma collected from behaving such a way cannot be erased just by applying the opposite. When he just stopped himself from being angry, all he did was creating less holes... not repairing them.
So, i think it is a beautiful story that can teach us all a lesson or two...