Yes I agree with the above, you are getting confused with the legislation on birth registrations and individual choice. My grandson has both his parents surnames and he used either or both depending on what he has to do, for day to day school he just uses one as it is easier for the teachers, for his passport and his building society account he has to use both as he used his birth certificate to get them, he'll either carry on using both or drop one of them, to be honest it doesn't matter, but to you as a grandmother it will seem desperately important, your son needs to get this sorted out and the mother cannot refuse to allow the child to have the father added.