venture, I feel your pain. My older son started asking such questions at the age of 3-- he is a very skeptical little one and always has been. If I recall correctly, the very first reason for the questions was the mall Santa. My wife was the one who wanted to draw out the fiction in our case. Like you, I feel strongly that I should always be truthful with my kids.
We were having a conversation one day not long after that about lying; he had lied, I don't remember about what. I was explaining to him why lying is bad, and at one point told him I had never lied to him, then modified it slightly to say something similar to "Well, I've never lied to you about anything except to avoid hurting your feelings". Of course he had to get to the bottom of it then, and I told him. He was happy to know the truth and actually thanked me.
I will warn you that telling a little one about Santa may cause a backlash, and you may get a lot of judgmental comments from family and even total strangers.
The truthfulness issue has also caused us some stress regarding religion. I and my wife can't tell our son that we believe, but on the other hand don't want to discourage him from being religious either, and certainly want him to have respect for religious beliefs. You walk a fine line in dealing with these issues with a gifted child, and you walk it when your child is much younger and perhaps emotionally unready to hear the information.
I would tell him that the presents will still be given. If you think it will reassure him, tell him that misbehavior will still earn him a lump of coal.
