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We suggest that you request a copy of his Official Military Personnel File (OMPF). OMPFs and individual medical reports for enlisted men of the U.S. Army who were separated from the service after October 1912 and before 1959 are in the custody of NARA's National Personnel Records Center (NPRC) in St. Louis. In many cases where personnel records were destroyed in the 1973 fire, proof of service can be provided from other records such as morning reports, payrolls, and military orders, and a certificate of military service will be issued. Please complete a GSA Standard Form 180 and mail it to NARA's National Personnel Records Center, (Military Personnel Records), 1 Archives Drive, St. Louis, MO 63138-1002. Veterans and their next of kin also may use eVetRecs to request records. See eVetRecs Help for instructions. For more information see Official Military Personnel Files (OMPF), Archival Records Requests.
We also suggest that you try searching on the military records website Fold3 for possible records regarding his service.
You may wish to search Ancestry.com or FamilySearch.org for the WWI Draft Registration Cards. There may be a fee for using Ancestry. Instead, please check for access at your local library as many library systems subscribe to these sites, making them free for their patrons. Please note that the World War I rosters have been digitized and are available on FamilySearch at https://www.familysearch.org/search/catalog/3378445?availability=Family%20History%20Library
We hope this information is helpful and best of luck with your research!