If he was never married to the mother, he would have had to be at the registration of the birth for his name to be put on the birth certificate.
Assuming your husband knows where the child is living, I suggest he contacts the local council's adoption department for advice.
If he was not married to the mother and there was no formal agreement, then he has no parental rights whatsoever and the child could be adopted without his consent. It is different for children born after December 2003.