To me the reasonable thing to do is if you want to get out then get out, sell back your stake. If you want to stay in then stay in, work on the project full time or work out some new plan where all stakeholders agree to switch to day jobs and put this into hobby mode.
But if you just want to maintain a stake to collect rent, and demand cash that doesn't even exist, seriously F that guy. The only reason he's in the legal right to be this douchy is because they failed to write their 'pre-nup' properly, not because he's actually in the right and his position and expectations are reasonable.