This person gestated and gave birth to the child - they’re factually the mother. The birth certificate should record this. Perhaps there could just be some kind of ‘add-on’ or attachment saying this person now identifies as male or it couldn’t record both their names or something.
Very few people will see the BC and if Freddie is a transman, they’ll be treated as such in society. If anyone were to quiz them about ‘the child’s mother’, could they not explain the circumstances? In reality, most people wouldn’t ask. A man collects his child every day from my DD’s school. I wouldn’t ask him where they child’s mother was. I doubt most casual acquaintances would ask Freddie.
As a PP said, if Freddie could cope with stopping the testosterone and being pregnant - a huge indicator of being a woman - then a category on a BC that they’d rarely look at would surely be Ok? Or is it all about erasing their past? Or de-sexing every single thing in society?
‘Mother’ is so much more than a social construct.