Hello, i have a 9 month old dogo argentino, Beau. He has been sweet and gentle to people and other dogs until recently. He is suddenly very aggressive towards strangers especially all men. He snapped multiple times on the behavioral therapist. He has not bitten anyone YET, but we are now forced to keep a muzzle on him. He has been timid from the start when we got him at 5 months old. He has never been aggressive towards myself or boyfriend... any advice?