Talk to a dog trainer.
A bit far away
but my niece deals with problem dogs. She includes them in her pack for beach and bush walks. A big Tibetan mastiff is the boss, the Alsatain is the brains, the list goes on - about five dogs. If the visitor dog shows any agression he gets a whopping. In a short period of time he learns he is not the boss, big, smart, a fighter, and if he thinks he is; something really bad is going to happen to him.