I wanted to message you as I was in your current predicament up until last week as our daughter decided to up sticks at 15 and moved into a house which was completely unsuitable (woman allowed teens in and they had a free rein to do what the hell they liked).
I called SS after a week of her not living here and by that time she had turned 16 and thus,a difficult age to legally do anything about where she was living.SS listened to me about my concerns and were sympathetic but offered no real concrete help.
I made sure her school was well aware that she wasn't residing at home and if she didn't attend school,it was because this woman wasn't enforcing school attendance.I certainly recommend you speak to your sons head or deputy and explain the situation fully-they sent me her lesson timetable,exam time table and called when she didn't attend school so I was in the loop (and also it was back-up evidence if I needed to report this woman for lack of care).
The main thing we also did was to refuse to fund her in anyway so the woman had to fund her food,clothes or whatnot.I didn't think any money we gave would go on worthy things so refused.It seems that is one of the reasons why she ended back at home last week.
It turns out I was totally correct in my assumptions and worries-she came back home last week with a stinking attitude (swearing and verbal abuse and EVERYTHING IS MY FAULT!) a tattoo,underage drinking and there are rumours of her sleeping with some random from FB.
I have called SS again and they are finally coming today with a view to help us all,I'm not holding my breath to be honest but it's getting to the stage where if her behaviour doesn't change we will have to think about her going into care as I'm getting mentally exhausted,dh is on the edge and ds who is 8 cannot bare being in the same room as her due to her bullying and angry outbursts.
Do give SS a call,I'm not saying they will help for sure but if he is going to come home at some point (and he will)-you'll need a network of help.
Do you have a behavioural support worker attached to the school?
We do and I have made them also clear of the current situation at home and how bloody tough we are finding it and daughter is also being disruptive at school so it's clear she is pushing boundaries in all areas of her life.
Fab time to start screwing up isn't it? GCSE exam start next week for daughter 
Feel free to Pm me at any time (I don't sleep much at the moment through the stress of it all).
Thoughts x