You're welcome OP.
I'll also add that your family is worse than I thought. They are so inconsiderate towards your feelings and your son's feelings! I think you are doing right by not telling your son anything about it, because it can wait, after the court.
I understand how confusing the situation with your family is for you. The moment your family invited you to dinner, without your son, they made you choose between your son and them. They had no right to do it, and it's a terrible situation to be in. A loving family wouldn't do it, especially now. I agree with you, is better to distance yourself from your family for now, because your priority is to support your son.
I think your son, like all victims of coercive control don't fully understand what he went through. That's why he doesn't want to talk with you, the police or a solicitor. He feels guilt, fear, shame, and thinks it was all his fault. I wouldn't be surprised if your son was threatened that if he'll speak, you'll pay for it or his siblings. And he's trying to protect all of you, that's why he's not talking with the police or solicitor.
It's hard to understand coercive control and you need to make sure your son understand what it is. There's plenty of YouTube videos, documentaries (BBC, Channel 4 etc.) and books about it. You can suggest him to watch or read about it. Maybe it will help him to understand how it works, realise he's actually a victim and he'll start talking about it.
I watched a documentary a while ago about how gangs coercive control boys from the age of 10-12 to sell drugs. It is a big problem in the UK, it is coercive control and no child deserves to go through it. There's plenty of sexual abuse (including boys, not only girls) and physical violence. I don't want to scare you, but you may want to try and talk with your son about it too. People who are involved in it, have no morals, are really dangerous, narcissists, sociopaths, aren't scared of police etc. Basically all of them are drug addicts, who don't care if they live or die and who they'll hurt in the meantime.
You'll need to support your son a lot to go through it. Also, make sure you and your other children are safe. 💐💐💐