So the contract is left at "changes requested" but I can't really make any changes as I don't know what he wants.
What is the best next step?
First of all, let's get one thing out of the way.
I don't think your work was "just awful."
But it DOES NOT MATTER if it was awful.
This is a fixed-price contract. You were hired to do a specific task. If you did that task, then the client needs to pay you for your work. That's what a fixed-price contract is.
If the client doesn't appreciate the quality of you work, then it is his right to not hire you again. He can even mark you down in the categories in which he felt you were lacking, when he gives feedback. But he can't simply "not pay you."
You asked: What is the best next step?
If he requested changes, but did not specify what changes to make, then you need to make a change and re-submit. Think of one thing you can do to improve the work, make the change, and re-submit it along with a note about what you did.
You can always think of SOME change to improve your work. Even if that "change" is simply providing a more detailed description of your process, the work that went into the task, etc.
DO NOT contact the client through messages. DO NOT click the submit button again.
You want him to either DO NOTHING or release payment.
If he is really irate about the project, maybe he will do nothing. That is FINE. That is his right. If he does nothing, you will be paid automatically for the work.
If he clicks the button to request changes again, but doesn't tell you what to change, then his behavior is very questionable.
But in case he is not simply trying to get free work, you could ask him if the two of you could come to some kind of compromise. For example, he could release half payment, and you would accept that, and in exchange you would retain ownership of the work.
If he can't be reasoned with, and if he just keeps clicking the "Request Changes" button, then you may need to file a dispute in order to get your money.
You have to resubmit the work to start the 14-day countdown for payment again. If the client then refuses to pay, I believe your next step would be to initiate a dispute.