This is a horrible situation for the OP and her son but I'm really not sure how much can be done without the buy in from him which at the moment they don't have.
Him engaging will be scary. He feels like he needs this relationship at the moment, even if there are problems. He will possibly be scared of her and her reaction, he might not want to admit he's turned his life upside down for nothing.
He might not feel he can come home so he needs to know they aren't the only two options available.
To look at a CP plan they are going to have to clearly evidence actual/risk of significant harm and it doesn't seem like there is enough evidence of this.
I mean, they're not stupid and they'll all be able to see the situation and see that it's really not good but actually they need evidence. That doesn't mean they don't care and don't want to do anything but they're right that 16-18 is really tricky.
Staying on CIN for another review period is about the best they can hope for to keep an eye on him for a bit longer.
I feel like he needs someone to come alongside him that isn't a counsellor or social worker and really get to know him and build that trust with no agenda and no judgment.
Maybe a youth worker or peer support and something that isn't time limited or dependent on social services intervention. Someone to say, I'm not going anywhere and someone that won't give up if he initially tells them to fuck off.
He is going to be surrounded by judgment at the moment and he and the girlfriend will retreat from everyone to avoid a light being shone on their issues and the nature of their relationship.
The suggestion to get her arrested and force them apart would almost certainly have the opposite effect.
He likely won't cooperate so again, they won't have evidence to prosecute her.
It will also drive them together, them against the world and strengthen the bond between them.
I know I sound really defeatist but I feel like playing the long game is the only option here.
Trying to keep connections with him, find him a trusted person to get to know him and listen to him or just kick a ball about a couple of times a week.
Be there and hopefully he'll have the strength to speak out and ask for help if he needs it.