I (male) was in an abusive relationship with a woman back in my twenties. She was always, I can recognise in retrospect, pretty controlling, but it took time for her behaviour to ramp up to physical abuse. We were together three years.
It took three separate incidents of physical attacks, including a final one which also involved threats with a knife (the police arrived before that got worse), before I came to my senses and EVEN THEN I felt sorry for her and actually felt guilty that the police had arrested her.
Unless things now are a LOT better with how the police treat male DV victims, then you’re going to face a lot of pushback from them in dealing appropriately with this. God knows, women have enough problems getting help in DV situations and that’s despite all the focus and training on it. My experience with the Police would suggest they’ll bend over backwards to avoid helping your son. So you’re going to have to push hard and advocate for him.
I had already reported injuries to the police (the second attack) in person at a police station. They literally couldn’t have been less interested in helping me. I got some vague waffle about taking out an injunction (at my own expense and privately) but no other help or guidance and they didn’t even take my details / report the crime. I was naive then - these days I’d have insisted on a supervisor and would have put in a formal complaint but back then I walked away and assumed it really couldn’t be that serious and I obviously just needed to ‘man up’ some more.
Explain very clearly to the police that you want support in evicting her immediately. Make sure that you are VERY clear that you think she’s a threat to you and your son and that she may also pose a risk to herself (the threats to self harm). Do NOT let them duck their responsibility to you, your son or to the GF. Unless you’re lucky, they will take the path of least resistance - whatever is easiest for them to get off your ‘job’ and on to the next.
Also tell your DH. He deserves to know what’s happening to his son and you will need his support. If your son has a very close friend then get them involved too, if you think they’d be sensitive enough to help him / talk through the Stockholm syndrome stuff.
Yes, your son will probably resent the ‘interference’ at first, but you have to ignore that in order to protect him (and yourself / your home).