You could probably check to make sure all of the fields are valid before disabling the button.
Are you using the webassist jquery form validation? If so, then it would be something like:
onclick="if ($('#form1').h5Validate('allValid')) document.getElementById('submit').setAttribute('disabled','true');'
You would have to give your <form> tag an id="form1" attribute and add the onclick event to the submit button.