I think I would handle it like Signmeup said. I wouldn't push the concept of Santa being someone that we believe really goes around putting toys under trees in people's houses on Christmas Eve. I would treat it more like any other fictional character, like Barney, Mickey Mouse, or Elmo. Now a days, kids are so used to seeing characters on TV and even in person that it wouldn't be hard for the child to think that Santa is just another character from a storybook or TV show.
The problem, I think, will be when they come back after Christmas and start talking about the toys that Santa left them under their tree and that child sitting there wondering why Santa didn't leave him anything under a tree. That is, if he's old enough to understand what they are talking about.
I also have realized that a lot of people have stopped telling their children that Santa, the Easter Bunny, Tooth Fairy, etc. are 'real'. Honestly, I don't know how I feel about that. I think it's sad that the child isn't allowed that fantasy of magic and wonder. But at the same time, as long as the child is told the truth of what started the concept of the character, and how we now carry on the tradition, I think, maybe, it's okay to not let them have that fantasy.
This link tells you the history of the Easter Bunny.
http://news.discovery.com/history/wh...ter-120406.htm
Here's one for Santa Claus
http://www.unmuseum.org/santa.htm
And here's one for the Tooth Fairy
http://www.fountainviewsmiles.com/hi...oth-fairy.html
However, I'm not quite sure that you would want to teach the history of the tooth fairy to a child until they are well into their teens!