It still really sounds like a bad idea. His intentions are primarily to use the product so at some point it's going to to deviate from what he wants to a more general solution for others.
Maybe he should be an advisor, maybe he should be an angel if he's qualified or have some amount of equity but not an ongoing role etc, or maybe he should just be billed for the work that he wants.
He definitely shouldn't be your boss or anything even approaching equal partner in this venture, and you have to look at the big picture... you could still be doing this crap in 2018.