"The 3rd co-founder and I find it demotivating and frustrating when he disappears." This seems like a bigger issue to me than the one in your subject. His dis

"The 3rd co-founder and I find it demotivating and frustrating when he disappears." This seems like a bigger issue to me than the one in your subject. His disappearance should not affect the validity of your vision and the validity of the opportunity. If his disappearance is demotivating this is a sign that your motivation needs work. If you fire him something else will replace him as the conditional hook that will trigger the demotivation. You can fire him or keep him. But your reason should not be that his presence is demotivating. Or rather if this is your reason you will find yourself demotivated anyway, regardless of what you do.