I think the fact that people here don’t seem to know the difference between Clare’s law and Sarah’s law means that nobody can possibly tell the OP what she can or can’t find out.
Claire’s law is for adults to be able to do a background check on a partner to see whether they have previous convictions against a partner. Nothing to do with child abuse and only a partner can request one.
Sarahs law is to check whether someone has a history of child abuse, someone can request one if their child is in that person’s presence, but the information from it very much varies from area to area.
I do also agree with @Nimbostratus100 · that the three year old’s wording sounds indicative of something else.
There’s no way a three year old is going to be able to tell someone word for word exactly what happened several months ago and be accurate about it. That doesn’t necessarily mean that he’s lying, he’s at the very least heard the wording from somewhere, but “hit around the head” isn’t three year old language.
OP does he go to nursery? Could he have heard this wording from another child who has heard it from a parent or older sibling?
How did the conversation emerge when he told you all this? Did you lead him in any way?
I’m not saying that this R is innocent, nobody knows. But talk of offering ultimatums and threatening the MIL that if she doesn’t break up with him she’ll never see the child again is over dramatic. It’s possible to minimise the child’s interaction with this man without having to go in all guns blazing based on an account of a three year old three months down the line using language which is not a child’s language.
I would be more concerned that this has happened somewhere else tbh or that he’s overheard someone telling someone that it’s happened to them.