Exactly what you should do depends on a lot of details about exactly what your business setup looks like, what your personal situation is, and what the other parties to the situation think. Way more detail is needed than you'd provide to any of us internet randoms. A good lawyer who understands these kinds of contracts is somebody you can actually give all of the important details to and who can give you accurate advice for your situation. This is not a good time for some weird quirk of your contract or any related laws to surprise you. If your co-founder is being unreasonable, you need to know what your escalation options are, and you may need to communicate that you are prepared to execute on them promptly if they don't give you what you feel is a reasonable deal.