In your position I would assume I have rights of ownership and act like it until I was told definitively otherwise. He put in money, but you put in time and expertise, which is also a high-value thing. He can start a new company but he can't sell something he doesn't own. And if he does, keep a tab on the new company. If they do well, you may be able to go after them for being built on software you partially own. Don't just flip over and shrug.