I would assume the name he changed it to. I have that anomaly in my tree that appears to be a brick wall, without a little explaining and sourcing.
In another case, I'm only a Smith because my dad was. There is some dispute that my 5th great grandpa may have been base born and given the Smith name. At some point, I think your gr grandfather's name would be associated to his father's surname, probably to his disliking.