My wife and kids have been re-watching the Harry Potter movies. The snob kid, Draco Malfoy, has a marvelous sneer in his voice when he talks about Ron Weasley's family. "They let you in? Really? A Weasley?"
Point being, it just occured to me that when the Nazarene Jews said "Isn't this Joseph's son?", that was as close as they could get to saying his last name. Not that they said it with any special disdain, but simply in shock that the son of a carpenter would suddenly be so impressive at teaching from the scriptures.
Back in my home town, I can think of a few things shocking enough to make people say, "One of the Heroman boys did that?!?" Not that I'm sayin what. Just imagine something either really good or really bad. ;)