Consider from the cofounder's perspective: Say he does sign the vesting agreement. Things are going well, but you've taken on more investment, and those invest

Consider from the cofounder's perspective: Say he does sign the vesting agreement. Things are going well, but you've taken on more investment, and those investors insist you drop the cofounder hours before his vesting begins, even though he's been a good contributor. It's a cheap way to enhance their equity position, and now that the company's survived to infancy, the cofounder's large looming equity stake is a liability. This doesn't always happen, but it does happen sometimes. Consider a shotgun clause instead of, or in addition to, vesting (strictly for founders). Also consider voting structures that disproportionately favor founders, so that he can't get voted off the island even if he's not financially vested. I don't know your situation exactly but this sounds like a struggle of control rather than wealth.