Yeah if you can't explain something he doesn't believe it. When you say he'll understand something when he does physics in high school he gets grumpy about being made to wait. He questioned Santa when he was 6 because its not possible to get around to every house in the world even with the time difference extending the night time.Smart kid. You should be proud that he doesn't get it.
I let them have Santa and the solstice celebration because its all about the kids, but I don't expect it to last much longer before they work it out.