As an early employee, his strike price should be low, so the cost of buying his stock shouldn't be prohibitive. I'm not sure about tax laws, but I'm very sure that paying some tax should not be a reason for OP to stay where he's being overworked and underpaid.

I doubt the tax burden is so bad, but if it is, then the decision to join the startup in the first place was even worse than it seemed before. OP should just treat it as a sunk cost, cut his losses and leave.

Staying just escalates his investment and near-certain losses.

I haven't worked for startups in a while, but if the tax situation got to a point that it's compelling employees to stay put, then working for a startup is even less sensible than I thought.