What is Jean Claude's last name?

as far as cannon is conserned he is never given one. and historically he would have probably been indentified by either his trade or the town he came from, and since we know niether of those we can't even guess at what it is.

Truthfully, we don't know much about his life as a human.

He was the whipping boy to a noble's son.
He knew his mother though never mentioned a father.
He had a younger sister.
He was a "wanton thing".

That's pretty much it.