Something like (obviously don’t use these exact words) “Hey Jim, I know it’s a drag that I’m leaving the company like this. I’m sad I have to go too. I know it’s messy if I retain equity, but is there any way you could pay me a buyout to reflect some portion of the hours I’ve put in so far? I’m pretty broke and it would make a huge difference.”
Then if Jim honestly feels that is fair, and he (very big consideration) feels like he can actually afford it, he might just do that.
The number, in that case, will probably be lower than you imagine. Because everyone tends to value their time highly and other people’s time less so.
But it would sidestep any fighting, and Jim will feel less resentment because it was genuinely his choice.
Or he will say absolutely no. But again that’s what you’re likely to get no matter how much you fight, the only difference being the damage done by fighting.
And in either event, whatever Jim does, you can focus your mind on your next thing in life.
Edit: Because remember, starting a company always had the risk you would work a whole bunch and end up with nothing to show for it.