Well, that's one of the cultural quirks I was talking about: the Mr. and Miss thing. It's probably never going to come up in the RP so I don't mind talking about it. Billy is respectful and polite, sure, that's just how he is. But the thing where he never refers to anyone by their first name is a cultural thing. In the Millennial Kingdom, age is important. Very important. Grandfather is the oldest thing in the kingdom, and the ground the lich walks on is worshiped. The older something is, the more it is assumed to be worth and respected.
To refer to someone older than yourself, even by a second, in a casual manner without their express permission is not only a social misstep, but a grievous insult. To the point where, even if two people hate one another and get into screaming matches in the street, the younger of the two will still refer to the elder in a formal manner. To not do so is basically a declaration of a blood feud.
Getting back to Billy, he doesn't know how old any of you are especially considering your all from different dimensions. So he is formal with all of you, just to be on the safe side.
Hey son, wanna' learn how ta' make witch balls?