Just ask him what money is to him. "What is money?" Is the first domino. I used to not know either. It wasn't until I figured out that the money we use today isn't "real" that it clicked.
Discussion
Could be the right approach. Recently I heard someone say 'you dont want money you want the things it can buy you' and then if you add in inflation it means that you get less goods and services over time ie less of the things you want.