You've sort of set it up to sound unethical. If you framed it as collaboration on a not-for-profit project that sounds fine. But you're framing it as "making him work" and giving minimum hourly requirements. Why?
Thanks for replying and pointing out the framing. I am not going to set minimum hourly requirements but I also dont want him to work 3-4 hours a week and expect to see results. I said "making him work", because he will be ultimately working on my idea. But yes, the wording could be better. May be "collaborate " on my idea or some such thing