Paul,
My sympathy is with you.
You spend the money developing the hull in the first place, have qood quality control AND reputation, then this feller wants to make money off your back.
Yes his product looks sub-standard, but if he improves, then YOU are likely to take the flak for a product that isnt even yours.
But
I do see a problem PROVING it is one of your hulls, especially as yours is the correct dimensions, so will his be.
Best of luck whichever path you choose
Bob