Your son has asked a deeply philosophical question. Good for him!
Truth of the matter is, those with less are often perceived as less worthy by society. And, a lot of people, and a lot of cultures, over time have grappled with the discrepancies of economic classes. Most religions address the issue, whether it is described as karma or "The meek shall inherit the Earth." Your son is starting to make his own theory of how all of this works.
What do I have? How did I obtain it? What do others have? Did they "earn" what they have? What is fair? How do I feel about myself when pondering these issues?
One year I made a lot of contributions to charities at Christmas time. I had told myself that since I had so much that particular year, I had enough to share. With some reflection, I found that there was an issue in my life that I had been feeling guilty about (and with a few years behind me now, I see I had no reason to feel that way!), and that I was trying to find redemption. I did make me feel better, but I needed to work out the issue to dispel the feelings.
Perhaps you, by chance, feel a bit guilty in general, and that's why you were taking your son to show him that giving is important. You are doing the right thing, but your son happened to stumble upon how you are feeling, and his comment brought up those feelings to the forefront. Breathe. That's okay. They are there, and now you can acknowledge them and just notice them. At some point, you will know what to do to bring relief to yourself regarding those feelings.
The belief in Santa is a gift parents can give to their children. It is a gift of imagination and fun. People rich and poor participate. How it is celebrated in one's home demonstrates the creativity of the parents. It need not be expensive, and it need not be "I bought you this; there's no Santa."
The manner in which the charity official was thanking you didn't hit the mark. It seems like a feeble attempt to have people donate even more since you have already shown your willingness. Perhaps in the future you donate without staying for the "thank you."
It's becoming more and more clear to me that the "He knows when you are sleeping" is to help with the hyper behavior kids have at this time of year. Wow! Talk about excited! And hard to go to sleep! And hard to sit still at school! It almost seems like a delayed bribe. It could also tie into the "Good=heaven; bad=hell" dualism. Behave or you're in trouble. If you don't like that part of the Santa legacy, eliminate that aspect.
At some point this will evolve to, "We (people) are Santa. We are being Santa when we buy gifts and perform random acts of kindness for others."
When he gets older, you can explain how the idea of Santa was taken over by advertising executives to sell products.
Also when he gets a bit older, you can introduce him to fairness in society and point out how some in society do treat the poor as "naughty" by the kinds of conditions that are placed upon them with taxation and other laws. I plan to be the person to explain social injustice to my kid.
In the meantime, enjoy your role as Santa. It is not your fault not everyone has all of his or her needs met. You have given your son the gift of giving to others. Take care of your sphere of influence.
Merry Christmas.