Thank you for your question
My name is ***** ***** I shall do my best to help you
Your son has Parental Responsibility for the child and the right to have the child in his care.
On that basis the Police cannot take any action.
However if your son is willing now to commit to the full time care of the child and wishes to prevent any risk of his ex removing the child from his care then he will need to make an immediate application to the courts for a Child Arrangement Order stating that the child lives with him.
This will ensure that if she does take the child from him then the police will help him recover the child
The form that he needs is here
and the fee is £215
Please ask if you need further details