As everyone else has said, it really depends on the actual program, and what your agreement was before. What rights does he have? Does he have the right to the code? To distribute a binary? Who owns the copyright of what you've already written?

If you own the code, and he's distributing, then you've got a let to stand on. If you're the only one who's every going to be able to make the program, you've got a leg to stand on. If if you own the code but he owns the concept, and he could hire a contract programmer to reimpliment the program, you've got to negotiate some figure under what he could pay someone to redo all your work.

Matthew