Thank you for contacting History Hub concerning your grandfather’s service.
First, 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 searched the National Archives Catalog and located 94 series in the Records of the Adjutant General's Office, 1917 - 1985 (Record Group 407) that are part of the Philippine Archives Collection (PAC). Some of the series have been digitized and are available via the Catalog. For more information about the non-digitized lists, please contact the National Archives at College Park - Textual Reference (RDT2) at email@example.com. Also, for information about PAC, please consult NARA’s website About the Philippine Archives Collection.
Best of luck in your research!