We have the Santa lie we tell from a young age, and from there, once the child realized Santa isn’t “real”…and it’s just the parents buying presents…well, now we (possibly) have a materialistic child that thinks adults lie about everything unseen.
I think in resolving to not lie to a child about something as major as Santa might help benefit their belief system, in that nothing comes crashing down before they reach adulthood.
But in hindsight, after such damage may/may not be done, you might consider researching some books by Saints that your child is capable of reading (whether short enough or with language relevant/understandable for his age). …after reading a reply of yours, if you have a theologian-in-the-making, you might want to have him delve into more theological Saints’ books.
I will note: when I was a child, I loved hearing about all the visitations to children by Our Lady…unfortunately, my parents weren’t well aware of all of these, and it was pre-internet/information age.
Always be honest with your children when you answer or consider answering, so that if you do not know how to answer a question, then you are able to say so.
Also, consider: if there’s no God or afterlife, there’s no harm done, but when one finds that there is a God and an afterlife after all the harm they’ve done, they’re in a much more sore position.