I think Cricket's suggestion is an excellent one. Eliminate the thing that is causing him to draw an erroneous conclusion (for now).
I'd probably also make piles of pennies representing the money of several fictional individuals and have him calculate their "average" wealth and then redistribute the piles so that they all have the same (average) amount. Have to make sure you start with a total number of pennies that is a muliple of your number of fictional people.
Finally, I would use the word 'average' in conversation frequently, even if it's a situation where it's not exactly mathematically correct.
