There are three options (#2 is best):
1. Just not claim him.
2. Have him file an amended return, unclaiming himself (note my previous answer, his refund will [most likely] not be reduced). You do not need to wait until his amended return is fully processed, to claim him on your return. But, you cannot e-file. You will have to mail in a paper return.
3. File a competing return and let the IRS figure it out. There is a rule that says IF somebody else CAN claim him as a dependent, he is not allowed to claim himself. You cannot e-file. You will have to mail in a paper return.
