Have you considered that you can be wrong? The mode of business has proven to be sucessful for some but not the others (just like any other business). Can it be a bias against certain mode of business?
I am not sure if you have considered (for the business), working out the details of the business plan on paper in front of both of you togather and check for assumptions that you differ.
Follow by reseraching the facts (such as the stock level at home, reciept and payment records, nothing grand, just fact of your own), and have a reality check.
Present you case clearly and objectively. Ask if he will agree to the testing method on every stage before the test.
And yes, don't forget there are alternatives! There are alternatives other than sit idle at home (obviously)
For a character proof, if you would attend a church gathering a few times, and raise your question after the gathering (to him only). It will show that you are willing to make a concession, and your goodwill.
Then, you can offer him the opposite view, possibly from well known atheist authors and ask if he will also consider the other possiblity.