[Benholio]

You could try using a smaller, more simple deck to explain it to him, and see if he still thinks that way. For instance, Take 5 red cards and 5 black cards, shuffle them up, and "deal" 8 cards to imaginary players face down. Ask him what the odds of the top card being red or black are. Do the same thing only "dealing" 4 cards to imaginary players.
If the red/black thing doesn't match the whole "flop a set" thing as easily, maybe just play with 4 jacks, 4 queens, and 4 kings, or something.
If none of this dumbing-down convinces him, then commence plan b: Play this guy as often as possible.

Ben