A bully always comes back for more. You have now demonstrated to this buyer that whenever he says “I demand more free work” - you will obey him.
You cannot be held hostage though by the thought that he will leave you poor feedback. You might carry on giving him another 2, 5, 10, 20 or more free revisions - and he could still leave you poor feedback. He has already demonstrated that he is a bad buyer. All you will do is waste your time.
My advice would be to stop work, explain that you have fulfilled the order, explain he can pay for further revisions, and that if they continue to request free revisions you will report their account to customer support.
Sometimes you have to standup for yourself.