in my training, first displays of dominance by the dog (showing teeth, snapping the air, a growl) were always dealt with a bigger display of dominance from me. (aggressive posturing, etc are NOT tolerated.)
this has always been enough to nip it in the bud, because i show the dog that I am his BOSS and he is a dog and better dam well listen to me....
however, your boyfriend's dog has crossed the line. it has bitten both you and him. this is not something that has just "happened". the dog, through intimidating you, has learned that he's the boss, and you will give into him because you're afraid of him.
i don't deal with dogs who bite the hand that feeds them. they take one last walk behind the barn and they don't come back. because a dog who bites its owner, rather than protects it, cannot be trusted.
in all the dogs we have EVER had, i've only had to put one down. she was poorly bred though, and abused to the point of psychosis. i was her "last stop" because the woman who gave her to me had been chewed up and down by the dog. unfortunately, the dog didn't learn.
you really need to tell your BF that he can have you (and the baby!) or the dog, but not both. you're grown people. what if the dog bit your baby?!?!?!?