[lsiebert]

I'd talk to your fellow founders, then talk to him with them and express your concerns. Do the whole compliment sandwich thing (IE "You're clearly a genius, but we've been disappointed by your work output. We value your contributions and we wanted to get your input because we feel the current situation isn't satisfactory, and we want this company to succeed") Treat him as part of the solution, and at worst you'll be where you are right now. At best he'll take it as a friendly wakeup call.