My new dog is a monster! i need advice?

he is a bullmastiff 6-8 months. i read about them and know their stubborness and how yelling does not help then learn. i have 2 other dogs one of which is big as well (doberman)who i have trained very well with obiedence.I know hes just a puppy and ive trained my other 2 dogs so well but i cant help but admit that this bull mastiff scares me at times. i know he is just playing but i will be telling him to do something with a firm hand and he will play bite me, sometimes my whole hip or arm he will grab! or he will pull on my clothes and not let go.i know he would not try to hurt me but it seems he does this only out of rebellion when i try to put him outside. keep in mind..this is a BIG puppy and im a 5 foot 100 pound female. im ertive and strict with my animals but ive never had a bull mastiff. any pointers of what to do when he does not listen and clutches onto me and does not let go?