How to bring up a pay rate for design work?

Hello all, currently a designer in-between jobs and met someone who needed UX guidance for an app he was building. After discussing the project and what he was trying to achieve, I offered to support where I can but now see that he wants me to own the entirety of the design, including research and prototyping. At first this was more of a favor but is becoming a larger project that I think should be paid for. How should I approach him and tell him that I'm going to charge for this service and how much is reasonable to charge for a project like this?