ISO dog trainer who will work with us and our young, male mastiff. Specifically, he is a wondrous dog-- loving, playful, VERY bright and motivated to learn and follow commands. His vocabulary is amazing; his favorite is "go in the car". Now the problem. He is extremely protective of his family and friends, but very quick to go into "attack" mode if he is uncertain of a new person's intent. He hasn't attacked or seriously injured anyone, but we are at a loss to know how to train for "no attack". He's swift as the wind, large, and quite scarey if you don't know him; newcomers often respond to him with fear, which gets everything off to a bad start.. We love him and want him properly trained to wait for a command before he becomes protective. Seriously hoping for thoughtful training help.