Perhaps, to his young mind, he didn't understand what he was doing wrong, so he did something else to cheer himself up, and was never punished for it at the time (the behavior is very similar to the behavior in this question). Over the past few years after they left the father, his mother became aware of the behavior and did begin to reprimand it, but I'm not sure of the details; they are not yet comfortable/have not yet overcome the experiences, so I may be wrong.