He has shown to be a slow learner with respect to his ability to control Harry. In the last chapter, in fact, when he tries to stop Harry from accepting the debt. This slow learning is to be expected, because he's been able to control every other rebellious child he's had to deal with in N years of being headmaster, plus most of his political opponents, etc. And he believes that of course the Hero will listen to the guidance of the Wise Old Mentor.