I don’t know why your son cannot allow this because he is perfectly able to do so if he is named on the birth certificate because then he has parental responsibility.
Up until early last year, any application by you, a grandparent, to the court for child contact would have failed unless there was an existing strong bond between you such as of the child have lived with you for a long period of time or already had regular and consistent contact.
However matters have been to the court and it is now been granted and there is a proposal to change the Children’s Act in line.
Tell her that unless you get contact with the child, then you will be taking her to court for a child arrangement order and the court will force the contact.
The fact that you may not be your sons natural parent is irrelevant. As far as the law is concerned you are a parent you are the child’s grandparent.
Being blunt, the mother hasn’t got a clue what she’s talking about.
If you could please rate my service five stars (even if it’s not the answer you wanted) that would be really helpful. If you have any further questions you want to ask on this subject, I am happy to deal with those for you.