Board Thread:Questions and Answers/@comment-9431245-20130628182556/@comment-5674726-20130629001623

No surname was mentioned, that I know of.

It's probably the same as Gaius' - not that that was mentioned, either - since I think Hunith is supposed to be his sister, and the identity of Merlin's father wasn't public knowledge so he can't have used Balinor's last name, assuming that he had one.