If you've been paid, then they own what you gave them in exchange for the money. Assuming you've met your contractual obligations, you don't have to type one more line of code from here.

You should tell them the truth: you've reconsidered the potential of the project and you want to re-negotiate for continued involvement. They have the option of hiring someone else to pick up where you left off. If they exercise that option, then move on to another project. You probably learned something and made some cash.

If you ask me, the dad probably wasn't thinking "I'm gonna screw this programmer as hard as I can! muahaha!". It was probably more like "I want to make sure that my son doesn't pay some guy to make a website, then have him copy it and put up a competitor".

Just tell the truth and state your point of view and it'll probably work out fine.