I'm with you
I felt such compassion for you when I read your post. I have an 18 year old son, and it would tear my heart apart if he was doing what your son is doing.
As far as the restraining order - please trust the advice that you have already received about a having a protection plan in place (I work in the legal field). Do what you can to protect you and your family. that has to be first and foremost at this time.
His girlfriend should go to a shelter if that's what she needs to do to be safe. There are a lot more programs and resources available now then there were back in the day when my first "A" threatened my life. There is a reason they are there. Please take advantage of everything available, and encourage his girlfriend to do the same.
I know you don't want him to go to jail, but if he continues with threatening, abusive behavior that is where he belongs. Too many women are killed by their husbands and boyfriends to ignore the seriousness of his threats.
My heart goes out to you - please keep posting and let us know how you are doing.