We have an Aussie that is about 6 years old. He has recently become very aggressive toward the family. He snarls and growls when we get near him. He used to be the sweetest most gentle dog in the world. The change was very sudden, and we are now afraid that he will bite one of us, which is especially concerning since we have a 2 year old that plays in the back yard. What can we do to control that behavior?