Welcome to WebmasterWorld Guest from 184.108.40.206
Assuming you're both in the U.S., in absence of a contract stating otherwise, the creator of the work owns full right to it. If you want to own it, the programmer must transfer ownership to you in writing. The fact that you're compensating him for it through a percentage of the sales does not automatically grant you ownership. If you two have a falling out, he will be able to both sell the app himself and prevent you from doing so.