Just a business note.
I am not in the automotive business professionally but i am self employed and really doesent matter.

You know if you are satisfied with your work and the product you use.

The customer is always right may apply to making pizzas but not to a tradesman.

It is up to the customer to find out what products you use if they have a specific preference. Then they can decide on you or go elsewhere.
You cant compromise your business and thechnic unless use learn something new which we always do. however you have a product you know and have had success with.
Do your thing because when you change for those reasons you will always pay for it. ALWAYS, its the law or something.

Now if I could give you some advice,
I would stand steadfast on your paint and explain to him this is your product period.
Get totally caught up on your invoice then get a down payment on the paint job if you are concerned with him. Either way keep the car until payed in full.

If he is hell bent on dupont maybe you can work out something on finihing the bodywork and let him go elsewhere. On good terms. Even though most pro painters wont touch someone elses body work.

I know its easy to tell them to get screwed or something but the key is to get paid and make them think you are tryin to help them out.

Other than that dont waste your time , thats worth more than money.

Good luck