Where they married when your DSS was born ?
See below- copied from DirectGov
Who has parental responsibility?
A mother automatically has parental responsibility for her child from birth. However, the conditions for fathers gaining parental responsibility varies throughout the UK.
In England and Wales, if the parents of a child are married to each other at the time of the birth, or if they have jointly adopted a child, then they both have parental responsibility. Parents do not lose parental responsibility if they divorce, and this applies to both the resident and the non-resident parent.
This is not automatically the case for unmarried parents. According to current law, a mother always has parental responsibility for her child. A father, however, has this responsibility only if he is married to the mother when the child is born or has acquired legal responsibility for his child through one of these three routes:
(from 1 December 2003) by jointly registering the birth of the child with the mother
by a parental responsibility agreement with the mother
by a parental responsibility order, made by a
Living with the mother, even for a long time, does not give a father parental responsibility and if the parents are not married, parental responsibility does not always pass to the natural father if the mother dies.
If your DH has PR I doubt she has the right to move countries without his consent.