He needs proof that he was married to your mother. Affidavits or other evidence may suffice, or church records. You can check the documentary requirements using the US State Department reciprocity tables.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
Find out where, when and how they got married. Perhaps something was filed with the local courthouse and you could still get a copy - depending on where you're from, they keep those types of records forever. If it was a religious ceremony, see if anything was put in writing to evidence the marriage. You may need to provide secondary evidence such as witness affidavits, pictures, etc. but I would not do anything without the personal advice of an immigration attorney.
This advice does not form an attorney-client relationship and is merely informative. It should not by itself be relied upon to address a legal concern.
You must submit all required documents with the explanation as required for their absence during the application process.
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