Tell him "I am a pro and have used M/S paint for over 20 years. I am used to working with it and can guarantee teh results. I'll be happy to use any paint you'd like, BUT i cannot guarantee the results. Each paint and company uses slightly different formulas, and different techniques get teh best results. As I am not familiar with DuPont paints, I cannot guarantee the results - so if it sucks, you're paying anyway. BTW, I need to charge more for the DuPont paint as my local supplier does not carry it so i get no discount to pass on to you, i need to get the specific hardeners/thinners/etc for that paint, and need to clean my equipment before and after use to make sure nothing gets cross contaminated and causes problems.

SOOO, charlie, if you are willing to pay more and forgo a guarantee, then sure, I'll use DuPont or Krylon or sears best Latex - your dime."

See what that does.