Don't forget the Portable North Pole videos, where Santa himself speaks to them and knows what they have been up to and what they want for Christmas.
I think my older ones may have a suspicion that I make those videos, just from the way they word their request for them. "Can you get those videos from Santa yet?" rather than "has Santa sent me a video yet?" but it may be enough with a younger child. I'm not sure I would go too far to make him believe again, although I'd try gentle encouragement and like beanbabe said, careful wording.
My 6 year old DS2 really truly believes, he knows without a shadow of a doubt Santa is real as he has met him. He (BIL in costume) came to our house one Christmas Eve and gave everyone a present when we had family round. He even left a present for BIL who had to move his car because the farmer couldn't get his tractor through the access road
The following year DS2 woke up in the night and came and woke me up, he said "there are all presents down stairs but nothing in our stockings?!!" so I said "quick, go back to sleep, he must have had to go back up to his sleigh on the roof to get the rest!" DH quickly put on the costume and crept in and filled the stockings, DS2 sat up, his eye lit up, DH said "shhh go back to sleep". He legged it down the stairs to get changed and chucked the costume in the cupboard, expecting DS2 to be right behind him, but he actually went back to sleep. God knows how with how excited he must have been.
I hope that when he is older he remembers it as something fun and a special memory his dad made for him, rather than a trick IYKWIM
DS2 is the one most likely to question things like how cold the North Pole is and how is it possible, but he has seen Santa with his own eyes so doesn't feel the need to question it yet.