I see the biggest problem being that you don't have something to base the percentage you are due on since the guy won't give you the numbers.
That your services would be worth $x is of little matter since the agreement was not based on that.
I see ALOT of legal footwork needing to be done to pursue it.
In any event, here's a link to a guy I met on another forum; he specializes in internet legal issues. He may be of some help:
http://www.internetlitigators.com/