Should we believe him?

OP has said that the boy has told his mother of verbal abuse over 3 years and the digging the nails in the head.
The boy has also told his father that "Wendy" has kicked out at his leg.
If I read correctly the father brought this up with the mother when returning the child.
So the boy has told both parents!
The mother and father need to get together and discuss what their Dc has told them, then the father needs to broach the subject with "Wendy" to make her aware.
Of course she is going to deny it but she will be aware that the child is telling his parents and should stop her odd behaviour.
You say that you are worried that "Wendy" could escalate her bad behaviour if she is confronted.
I have related this post to my Dd who is a Health Visitor, she says that if you have ANY concerns at all regarding your son's safety with this woman, you must not trust her with his care.
She said this, quite firmly.

Very many thanks for all your advice ladies. I will take everything on board and go from there. Thank you all very much.

You can never tell with children, he may be speaking the truth or he may not. In any event he's not happy and something needs doing to help him. I should speak to SIL first and explain what you've been told stressing that even if untrue GS needs help and see what he thinks should be done next., then take it from there together.

The two parents should be discussing this , not anybody else at this moment. Find out what happened and then the father can approach his new partner about it.

I have to say though, sometimes when I'm at the hairdresser she gives my scalp a massage and it feels as if she is "digging her fingernails into my scalp". Not saying the child is making it up, but maybe what he felt was not malicious but just her scratching / rubbing his scalp, especially if she has very long nails as some girls do. Just a thought.

Always believe the child. Why would you not believe him? If she is hurting him it should not be allowed to continue. What is she going to do next? The boy will think there is no point in telling anybody if nothing is done. This is how abusers get away with it. Is it possible his dad could see him somewhere else where this woman is not around?

I feel concerned that anybody thinks there should be physical proof i.e. bruising before the child is believed.

What Quickdraw said ...... 100% agree

Please please always believe the child and look into this. If he is being dishonest then he’s crying for help and is unhappy. If it’s true then something must be done about it. Children do tell fibs about the parents new partners/ friends but there is a massive chance that he isn’t

jocarter spot on!!