Well, in my opinion there will not be any winners here, only losers (except for the lawyers).
But if you want to get what's yours (and to me it looks like a buy-out offer is the best that is in the cards) it helps if you have as much documentation as you can find, then visit a lawyer that will take your case on a no-cure-no-pay basis. Otherwise you might be out a large amount of $ just to find that you won't get anything at all (or just enough to pay your lawyer...).
It also helps to get an idea of what you think this company is worth. If it is worth more than several tens of thousands of dollars then you have to explain why you never had any kind of contract drawn up.
In the current situation, having an argument with your partner is actually a pretty good moment to figure out where you stand, after all he's still around to argue with.
Imagine how it would have turned out if one day he'd died and you would have had to prove to his estate that you own part of it...
Have a man-to-man talk, lay out the land for him, be fair and pay up what's a reasonable amount (and be reasonable) then go do something else, or alternatively offer to buy him out.
If there is no friendly response weigh your options and if you think it is worth it get a lawyer.
good luck, you'll need it.
This will take a lot of time.