As the father of an 18 year old autistic son, I know the pain that the family feels, as I've seen it in so many.
This family needs a bahavior specialist, and they need him/her right away. The anger they're seeing is not a symptom of Asperger's/autism, but of a child in a lot of pain. Many like to think of the rage as a symptom of the disorder, I think because it makes it easier for us not to feel guilty, but it's truly not.
Force isn't going to work, threats will only make things worse, punishing the behavior will make it worse still, only identifying the source of his pain (It's almost certainly mental, emotional, and physical.), by someone qualified to do so, and helping to mitigate it will solve these problems...
Please try to trust me on this.
Sending happy thoughts to your girlfriend and her family, as well as you. You sound like a really, really awesome boyfriend.