Yesterday, DS1 (6 years) and I are sitting watching an episode of Peppa Pig which tells the story of the tooth fairy. He suddenly pipes up that the tooth fairy isn't really, neither is father christmas or the easter bunny and that there is no point my telling him they are because he knows they're not and he also knows that I am all of the aforementioned characters (I am speechless at this point). He then proceeds to tell me that God isn't real either (fine by me, we're atheists anyway so I've already answered his questions on that one) but that he knows I'm not god 
I tried to convince (re the first three, fictitious beings) that they were real but he wasn't having it and said he know that if I try to pretend to him they're real, in a few years I'll admit he was right and say theyr'e not.
He has not been around anyone else (other than his peers at school) so unless it has come from one of his friends, I can't think where else he would get these ideas (other than the fact that he's right and presented me with a well reasoned argument to disprove the existence of father christmas at least).
My DH had a chat with him too, but the bottom line is, he doesn't believe and I won't lie to him and tell him they're real.
Was IBU to have done this? My mum thinks so and whilst I know 6 is young, what can I do if he's figured it out.