Actually, I'm going to bet that he owns 100% of the IP he wrote because if they didn't have good documents around something like vesting, they probably don't have good IP documents either. That gives him the upper hand in negotiation.
To the OP: without a vesting schedule or agreement on what happens when you leave, your partner is stuck with you. If you've done most of the work up to this point, don't be bullied or browbeaten into giving up your shares. I wouldn't even give up that 10% you were going to right off the bat—that's giving away something for nothing,
Most attorneys will have a conversation with you and give you at least some background and let you know what general options are. I've been through a situation similar-ish to this one and if it wasn't for the fact that my partners were so completely sloppy with their paperwork I would have been completely screwed out of several years of my life—they certainly weren't looking out for me. As it was, they took more than half of the proceeds when we eventually sold the company even though pretty much everyone but me had ghosted a couple years earlier.
> Jim can try to play games by re-incorporating and moving the IP around, but you'd still be entitled to 40% of it. But you might have to sue to get it.
Remember, if you didn't sign IP assignment papers when you founded the company, you own 100% of what you produced. Copyright goes to the creator in the absence of any agreement to the contrary. That also means Jim can't transfer it to a different company (which he couldn't do anyway without inviting a shareholder lawsuit.)
> BTW if you wanted a fair deal, you'd get 20% of the stock now and forever. That's what you would have vested in with a standard four year vesting agreement.
Agree with this—if you really think the company has a good shot, I"d negotiate a generous buyout (generous to your partner that is) of half your equity and keep the other half.