A few glaring issues here:
I hope your mom has sought care for the attack by now. You sound well aware of the dangers of infection and the chances of getting one from a dog bite is certainly a possibility.
This dog has had more than enough violent outbursts that it would probably have been put down by the state if the attacks were reported. This should put the dog's behavior in perspective for yourself and your boyfriend.
It's really worrisome that your boyfriend would prioritize his dog over you, especially after two years of dating. Seriously, he cares more for a creature that will likely die in less than a decade over a person he assumedly plans a future with? Think about that.
It's also worrisome that your boyfriend has had a violent dog for so long. He's obviously doing something wrong, even if that something is simple inaction. He might have trained the dog poorly, never talked to his vet about the dog's behavior, and seemingly has made little effort to correct this dog's behavior. What happens if you have kids with him? How long is he going to drag his feet if your children show signs of a disability or misbehave in social settings? Or what if some hardship fall on your relationship with him. How long does he drag his feet until he tries to fix things?
The last thing is that he never trusted you when you claimed that you and your friend were attacked. How do you feel about this lack of trust? It sounds incredibly disrespectful to me.
All I know for sure is that your boyfriend needs to make some major changes in his life. He doesn't sound like the kind of person I would call "life partner material" as he is now.
PS: Keeping dogs outside is cruel? What? Where does he think dogs came from? In fact, I wonder if some of the behavioral issues come from keeping such a large dog indoors all the time.