Your first mistake is not having a formal agreement written on black and white. Even a business set up together will relatives should have its own formal agreement. It does not mean that you do not trust each other, this is simply to prevent any future mischief. You are certainly in quite a bit of a situation, you may not want to go straight to your lawyers. You should try to talk to him nicely and work out a solution that is beneficial for both of you. Of course, both party should compromise. If this fails, I guess you would need to lawyer up and bring this to court.