You are absolutely right, this animal is trying to dominate you. If he is growling and biting when told "NO" he is certainly not playing. I own a brazilian mastiff ( Fila) He acted the same way,only he was younger, I hired an animal behaviorist to train him when he was 12 weeks old. And I will admit, I was horrified by how she handled my puppy, I wanted to take him and run. He refused to behave on a leash, she put a "prong collar" on him, and when he refused to walk, she drug him, when he bit her, she pulled up on his leash until he stopped the behavior, when he did walk for her, if he was getting "rude" ( ie, pulling, chewing the leash, running etc ), she tugged the leash and told him "HEEL" in a firm voice,when she stopped moving and told him to "sit", if he refused she tugged the leash (which tightens the prong collar) until he sat. Then she would give him a simple " good " and a pat on the head. After 20 minutes of this she handed me his leash and wanted me to do what she had just done,I was horrified ,and almost in tears, but I did it anyway-then she told me that with any dog, especially an aggressive dog or a Guardian Type breed, the only way the animal will respect you is if you completely dominate him. Not by abusing him, but by letting him know in no uncertain terms, that you are the only thing that stands between him and death and making him aware that at any moment you can end his life. I was horrified, but at the end of that one hour training session, my 12 week old puppy would not even so much as pee without me giving him permission. I never have had to raise my voice to him, he is now 17 months old and is 125 pounds and walks like a champion show dog on a leash, my 9 year old son can walk him on a leash without a problem,he is polite at the vet, the groomer, the flea market, all the places I was told I could never bring him to because of the breed, he can go without incident and without a muzzle,he is a wonderful animal, and it is because I showed him I am in charge. If it weren't for my trainer giving me that advice, I have no doubt that my dog would have been an out of control lunatic and would have probably already been put down.