Repeat the exercise several times. Make sure Mister is getting enough exercise. A walk daily or even twice daily is probably not enough. He must be able to run and stretch his legs. If he will chase a ball, toss it for him until he's panting mildly and tired.
Lack of exercise causes many problems.
My Australian Shepherd, Blue, barks at the mail carrier and the paperboy. He really seems to hate them. First, these people come daily, invade his territory and then leave. Blue feels that he must announce their transgressions and must warn them away. He also feels that he has won because each time he barks, they leave.