Is Withholding A Child’s Passport Illegal?

Some context : My partner has been going through a very difficult and lengthy custody battle the last 4 years, and the court has ultimately decided that he have full custody of his child (7yr). He wants to coparent equally, and is making all routes so that his child, ends up spending 50/50 time with him and their mum, something that she was not encouraging with him. It was decided for his child’s mental health it was best to move forward in the way described.
We are planning on a holiday in the summer, and this will be the first time his child has been abroad. The mum is refusing to give the child’s passport over to the dad, and says that her kid doesn’t want to go on holiday because they don’t like planes (despite never being on one..?).
As he has full custody, should the passport be automatically given to him? If she refuses, should he apply to the courts, and which side would have to pay for this?
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