Paul, let him believe what he wants - one day he'll come to realisation what the truth is and he'll be fine about it, no huge damage done. We're not talking religious indoctrination, we're talking harmless myths that make childhood magical. Ham it up even by leaving snowy footprints from the reindeer (flour/oats etc) and remember the brandy, cookies and carrots must be left out for Santa and his reindeer! He'll have those specal memories forever; no doubt one day he'll even tell his children all the silly things his parents did to help him believe Santa was real.
Even my JW parents perpetuated the tooth fairy myth - my father left me special notes with each 10p telling me what the fairies would be using my teeth for - I smile every time I remember back and bear no hard feelings toward my parents for stringing me along.
Childhood is so short, embrace the myths!