Well, I would refer to the Book of 1 Samuel 28. Saul consulted a medium to gain information (1 Sam. 28) and that information came and proved true (1 Sam.31:1-6). It is something that is real, but it is forbidden (Deut. 18:9-14). G-d forbade supernatural activity in this way; He only allows for contact with Him via dreams, the Urim and Thummim on the breastplate of the HIgh Priest and through the Prophets.
We know that there is a real supernatural world and G-d has established how we are to interact with it. Why do we tell our children not to play the weiji board “game”? Because it’s not a game, but real and we know we must protect them from it. I think G-d is like us in that regard. He forbids those things that that are real, not those things that aren’t. Does that make sense?
How you want to explain this to him would be up to you, but I hope this info. helps. It’a good and important topic.
Blessings,
Rachel