🚀 go-pugleaf

There is an old saying that goes, "The person who knows HOW to do
something will always have a job working for the person who knows WHY".
I say you should probably give him what he wants and be glad that you
have a product at all. If it is successful then the concept is worth the
price. If not, then neither of you make much and you can go on to the
next idea armed with more experience.

The only exception to this would be if you put up $$ to begin with. Then
the percentages are well established. Someone who risks $$ always has
priority over the person who just contributes know-how.

Wrap it up and move on to the next idea. Next time you might consider 
having all interested parties put up their share of the $$ that
would be involved in building the prototype or some reasonable amount 
to be considered earnest money. This establishes a firm percentage 
take of the profits and helps to ensure that everyone has something 
vested. The alternative to this would be to sell your services for 
a fee and give-up any take in the profits except for royalties.

Just my opinion. Good Luck!