His return will be rejected because you have already filed your return claiming your own dependency. You can discuss with him on whether or not to leave as is. Based on the parameters you gave, you are eligible to claim your own dependency as he cannot truthfully say that he provided more than 50% of your support.
You can amend your return and declare that you can be a dependent of someone else. I am not sure how long this will take the IRS to update your amendment since you are now able to efile those amendments this year. Until the update is effective however, his continued efforts to efile will be futile since you have already filed your return claiming your own dependency.
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