How to fire a co-founder with more equity than me

I have been working with three co-founders for about a year. Two are technical, including me, and the other is business. We have the equity split up on paper and have determined the exact price of each share. However things get complicated because the technical co founders decided to quit and we haven't decided how to dilute the equity if we ever buy it back.
After the co founder quit, we decided to ditch our original idea and work on a new product, where i developed 99% of the app. This is where it gets tricky. I have a buy out contract however the business guy doesn't, at least I can't find it on paper. Even though he claims to work long hours, he has provided very little value. His big role is to market our product, and he completely failed. He is the only one to invest money into the company and started it, so he has most equity. Now, he is show showing even less value than before, so I really want to get rid of him since I still have passion the new product. We haven't launched the new product yet.
TL;DR: We have on paper how much equity we own and the exact dollar value, but no dilution method. One co-founder quit. I have a buyout contract on paper but remaining co founder doesn't. Remaining co-founder with most equity is useless so I want to fire him. What are my options?