OP I’ve been here.
Social services will encourage you to take legal advice, they can’t give you legal advice.
But you should be aware if you know, and you do nothing, you could be culpable of neglect.
You need to take protective custody of the child. They do not go back to Mum’s.
You need to submit a prohibitive steps order removing her of her right to see the child and you should also submit an emergency C100 child arrangement order, IF not custody agreement exists.
You need to record and document every, single thing. And that’s because if Mum says she’s no longer seeing the man, then social services close the case and you are back to private court.
Even if she’s still seeing him, and lying to SS, you have to be able to document what is going on with evidence. You’ll need it for a later date.
I would recommend if she becomes abusive you phone the non-emergency police and ask to log it and red flag your address.
Be prepared that you may have to allow calls and contact. Be prepared that Mum is going to be an absolutely awful human to your step child, and take all the support you can get. It’s vile. Courts are backlogged and it’s a high threshold to take a child away from the parent.
Most of all, best honest with the child in an age appropriate way. Good luck. I don’t envy you, but it absolutely has to happen.