blue, sorry to hear things are grim for you too :(
You asked what happened when I had my son arrested...
Practically, it was all quite straight-forward: he was arrested, charged with assault and criminal damage, bailed to report to the Youth Offending Team (which was a way of getting a bit of support from them), we had a visit from them, he went to one session with them, he received a final warning, and that was it.
It might not be quite so straight-forward for your son - I don't know - because the assault was on someone else, and I think the victim now gets some say in what happens...? Also, what happens depends on the attitude of the young person. If they admit it and are sorry, the outcome is better for them. If they deny the offence, it will go to court. And I think that if they do not say they are sorry, they cannot have a final warning.
Emotionally, it was incredibly difficult at the time and for several months afterwards. It felt like a really big deal to me, and to him too - not something I ever imagined I'd do - but I couldn't see what else I could do, that would stop what he was doing. I was angry and desperate and scared. He was absolutely furious with me. He didn't come home for three days, and I didn't want him. I wasn't sure I ever wanted him home. I asked for help from a social worker friend, so that I could work out a practical way forward. I would have used a 'mediator' for the first meeting between us, but no-one was available, and in the end, I was able to be calm enough to handle it myself. But it would have been better with an independent person to do that for us.
I also went to my GP and asked for counselling, so I had someone to talk to throughout. It took several months for me to stop feeling I wanted him gone.
One thing that was good was that the attending police officers at the time were great. They were very calm and supportive. They were also clear that I had been the victim of domestic violence. I felt like they took it very seriously, and recognised how desperate I was.
One practical thing that didn't go so well was the time immediately after he was arrested and charged. I didn't think past the point of arrest at all, and I didn't think about what would happen. So I was completely taken by surprise when the police phoned me to say they releasing him to my care. I hadn't seen him for three days, and the last time I had seen him, he had assaulted me, smashed up lots of things, and waved a knife around, so I did not feel safe to have him home without support. No support was available. I refused to have him home. The desk sergeant was awful and judgemental, and called me an irresponsible parent. She also (outrageously) said something negative about me in front of my son, which left him with the impression the police thought I was 'making a fuss', and definitely made it more difficult for me over the next couple of weeks. :( (I did make a complaint about that, in the end).
The very best thing of all - the thing that made it all worthwhile - was that it stopped my DS from behaving like that. He had been kicking off and being aggressive and violent for many months, but after I had had him arrested, it never happened again. He got angry once, and began to lose control, but because he then knew that I would call the police, he kept control, and went out, instead of smashing things up.
Hope that helps somehow...