Me & my husband were not married when my son was born. He has my maiden name... but wants to keep it and add his father's name to it. Now that my son is going to have a son come May, he wants to add his dads last name to his.

In UK your son just needs to add his fathers name to his own. i.e. Tom Jones SMITH and to make it completely legal he can go to a solicitor and have it added by Deed Poll

I Live in New York

He goes to court for a name change. It's that simple.

As an adult it's a slam dunk. He files a petition in court and the judge grants it.

Try this.

Is it different if he wants to keep my maiden name and just add his dads? For example: his name is John Doe but wants to add his fathers name which is Smith... to make It: JOHN DOE-SMITH