Try just telling him "no" and see what happens. That sounds flippant but it will tell you exactly where you stand -- your value in this situation, in part because you didn't negotiate terms prior to building his prototype -- is exactly a function of your opportunity cost and his alternate options.

EDIT to add: Your framing and many of the other answers are mostly in terms of you and what you're "worth". I think you really have to reframe this in terms of what you are worth to the "company" aka the founder.