Totally have not read others' responses, since I don't have time, but couldn't resist replying because I had to address this issue very recently.
My daughter asked a couple of years ago if Santa was real. I said, What do you feel is real? He's the spirit of Christmas. Without Santa, there is no Christmas. (It's not a religious holiday for me; I'm an atheist).
This year, she asked again, and I know most of her friends know the truth. She was nine, approaching ten, at the time. I decided she needed the whole truth. But I still feel that Father Christmas is real, in the same way that other gods are "real". That is, they are invented by us, but for the person who believes, they do exist. And the spirit of Christmas really does exist in our collective minds, and it's important that it continues to do so, in some form or another. Santa really IS the spirit of the Christmas. He embodies a wealth of history that pre-dates Christianity and encompasses western cultural traditions. Whether you're Christian or not, midwinter is a time to pause from the craziness of life and share time with your loved ones. It is a time to give and receive, to relax, to be grateful. This idea is covered beautifully in Terry Pratchett's The Hogfather, so I read it to my daughter. The book is both funny and moving, but gets you thinking about what Christmas means. It took a couple of months to read, but when we finished, it provided a perfect starting point for a discussion.
I encourage you to read the book. It might help answer your questions. I too did not like the idea of lying to my child, but Santa is part of the magic of childhood, and The Hogfather gives very good reasons why it's important to nurture that early belief. Again, it's not a religious thing for me -- it's about our very humanity. We invent small lies like Father Christmas so that when it comes time to believe the big ones -- justice, mercy, compassion etc, the things that make humans unique -- children have an idea of how it works. We can't be human without believing in those things, even though they are merely concepts we invent and maintain.
Note: the book is totally inappropriate for a six-year-old, so even if you do read it, I don't think you would want to read it to him. But you can still glean some ideas.