Obviously the right answer here is to get a lawyer and ask them. In lieu of that for whatever reason, I’d confront the CEO and say you know about his actions and will spend the rest of the company’s money to sue him unless he returns $x, gives up all his equity, and resigns. If he declines this offer you should probably just quit and notify your investors and let them deal with it.

Be prepared to just call it quits, set everything to zero, and start over though.