My son filed his taxes and didnt put that someome could claim him as a dependent. I recieved my refund but my son's return always get rejected. what should we do?

I filed my taxes and received my refund on 2-22-18. on my taxes I used my son as a dependent. Last week my son filed his taxes and while filing he checked that no one else can claim him. Now his tax return is stating unacceptable and rejected. what shall we do?