I think it is too late now, i am sure that it could only be added within 1 year of the birth registration, however, there is no reason that the child cannot use his suname as a double surname, we did this with my grandson, though my son did go with the mum and have him registered together, and they gave him both surnames which is his legal surname. You should ring the register office and ask for the ruling on this, whether there is a time limit to adding a father, if within a childs lifetime the parents marry, and the father was included on the birth registration, the registration can be ammended to legitamise the child, but that's a seperate issue.