Aggressive Australian Shepherd

We have two one year old Australian Shepherds. Both are male. The smaller white one has bullied the larger black one non-stop. The black one has been a nice gentle dog. Last week the black one just snapped and attacked the white one. The white one cowered and the black one did not bite him. In response we gave the black one to a friend who was very fond of him.

A few days later he attacked a four year old boy who came into the house and bit his arm and back needing a couple stitches.

He has always been a sweet dog who is part of the family. Well socialized with other dogs and people. What should I do now as the friend who took the dog is very upset? Thank you for any thoughts.

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