Get a strong magnet. Tell him to imagine it is God. Show him there is an 'energy' around it.
Then get some pieces of metal. Maybe some keys or coins. Tell him they are us, the people. Far from God (the magnet) they are just there standing, not moving.
The bring the magnet near the pieces of metal but don't let them stick to the magnet. Some will stand up, all will more or less follow the magnet. These are the "followers of Christ". His students in childspeak. He gives them strength and life. They can do many things they couldn't without Him but they are still not totally with Him. These are, basically, all the people in other churches.
Now, let some of the pieces stick to the magnet. Now, not only they are as close as it is possible to be to "God", without becoming Him, but, if you let another piece metal that is separated to get near the piece of metal that is "soaked" with the energy of the magnet, they'll stick to each other. This is communion in the energy of God. The first piece of metal is a saint, and the second piece is a person praying to the saint. The energy comes from the magnet, not from the piece of metal. It's the magnet's energy, always. And (you move the second piece to stick directly to the magnet), the objective is that we all get together with God. And that in this way we become like Him with the same "powers".
Show also that the magnet doesn't move everything. Paper, wood, are not affected. That is not because the "power" of the magnet is not there, it is because these things are not of the right "stuff". When we are baptized God changes us from "paper" to "metal". But if we don't come close to Him, we don't acquire His "power". When we are chrysmated, we become pure metal, and when we confess we take the dirt out. When we go to Communion we reshine the metal and priests are guys who help putting small pieces of the magnet in the metal pieces.
You can and should improvise with these materials. Maybe, making faces on the magnet and pieces of metal or something like it. Use it as a little theater. The kid's imagination will do the rest. And you will have seeded his imagination with a sound analogy that will give itself easily to be deepened later in life.