Notwithstanding anything in the foregoing provisions of this Act, in the case of an illegitimate child, no person shall as father of the child be required to give information concerning the birth of the child, and the registrar shall not enter in the register the name of any person as father of the child except at the joint request of the mother and the person acknowledging himself to be the father of the child, and that person shall in that case sign the register together with the mother.