Oh you poor love, what a horrible mess. I think you need to let him carry on as he wishes. If you force him back home and he makes more allegations against you then what will happen to your other children? When he is ready, which may take many years, then start thinking about family therapy for him.
Has he been seen by CAMH or a Unit Clinician? There may be something going on in the background that he needs help with. If not, ask the families team to put in a referral.
What he is doing with his girlfriend requires specialist navigation. It is unlikely that you nor the girls parents have this specialist training. It sounds like the girl is in a safe place now, she will have been put with a foster family/home where there are adults with training on how to help her with being exploited. For a similar reason, I would encourage you to ask the families team why they recommend fostering for your son. I suspect they have a fostering team in mind who has some specific training.
Fostering doesn’t always go well, so I don’t want to raise your hopes too far. It is unlikely to make any immediate changes but may help prevent your son getting deeper into something he can’t get himself out of, thus limiting the longer term impact of his teenage years. It will also give you and your younger children a break from what must be quite a difficult time for them.
Lastly, I would still strongly encourage you to organise individual and family counselling for you and your younger children as soon as possible. The stress of this will be reducing your resilience and make you afraid of future parenting decisions (understandably), so you need to be able to talk it through and off-load. And your younger children will be confused about what is happening now and worried about what their teenage years will look like. This is step is so important, I can’t stress it enough. Again, the families team will be able to help you.
I am so sorry you’re going through this. It will feel manageable again, in time. Be kind to yourself and try not to stress too much (easy for me to say I know!). It does not sound like there is much you can do to influence the outcome of this, except get in ahead with your younger children to try to limit the future impact of this on them.