Hiya, Myself and my husband have been through the same thing, the only option that we had was the child had my surname on the Birth Certificate, but then we had to get a change of name deed done by a Solicitor. If your Son isn't working he maybe able to do this with legal aid. The only other option is that your Son adopts the child, in which the childs name should then be changed that way. Going through the change of name deed is what we had to do, although the name never changes on the certificate, they will provide you with a proof of change, and this will have to be handed in to schools and doctor's ect....
I hope this helps xx