My fiance and I want to change our name to his mother's maiden name. Will we be able to? It isn't his legal last name

I would go in person or even call to ask. That may be ok, but a technical grey zone.

Is better YOU can go in person we can say something and maybe is not like do you want

I believe you guys can, since marriage & naturalization are the only reasons you are allowed to legally change your name & you both can start a new family name right there.

You can go in person for more information