Offer him a buyout based on the first year's revenues. Or a flat fee, but delayed until you have revenue to pay for it. Or anything, really, but make it clear that you don't think he's as committed as you are to this, and you want to take the risk and gather the reward.

And if he says no, dissolve the whole thing and go find something else to work on. Odds are good you would have pivoted to something else anyway.