If you share the details with the group, along with where he lived prior to joining the Army (if you know that) someone might be able to look him up on Ancestry's U.S., Army Transport Service, Passenger Lists, 1910-1939 and other records that they have. It isn't his full service record, but it can often indicate which unit he was assigned to when he sailed to Europe, and which he was assigned to when he returned.
Dear Ms. Esparza,
Thank you for posting your request on History Hub!
We suggest that you request his Official Military Personnel Files (OMPF). OMPFs and individual medical reports for those who served in the U.S. Armed Forces and who were separated from the service prior to 1958 are in the custody of NARA's National Personnel Records Center in St. Louis. In many cases where Army and Army Air Corps 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. Navy and Marine Corps OMPFs were not affected by the fire. 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. If there is any information requested that you do not know, you may leave it blank, however the more information you provide, the easier it will be to locate the correct file. For more information see Official Military Personnel Files (OMPF), Archival Records Requests.
Due to the coronavirus public health emergency, the NPRC closed again as of November 7, 2020 until further notice. NPRC will respond only to requests involving burials, medical emergencies, and homeless veterans. If your request is urgent, please see Emergency Requests and Deadlines or emergency requests may be faxed to (314) 801-0764. Please check archives.gov/veterans for updates to the NPRC operating hours and status. If this is not an emergency, you may still mail your request now, but be advised that it will be quite a while until NPRC can respond to your order. We apologize for any inconvenience.
Please note that if there is a number on the dog tag, is most likely his service number. Please include that on the SF-180. Since social security numbers (SSN) did not exist during World War I, if you do not know his social security number or he never got one when those were introduced in the 1930s, you may omit the SSN without causing any difficulty.
In regards to unit information, where that becomes significant is if his file was lost or damaged in the 1973 fire and it is necessary to consult unit morning reports and rosters. You may submit your request without unit information, however if NPRC staff can not locate his OMPF they will send you NA 13075 Questionnaire About Military Service to request further information that will allow them to search the other records.
As was mentioned by the previous reply, the U.S., Army Transport Service, Passenger Lists, 1910-1939 available on Ancestry can often be used to identify unit assignments at the times that a soldier shipped to and from Europe. You may check with your local public or university library as these institutions often provide access to Ancestry.com. Also, if you share further information about the person you are researching, we may be able to search this database for you.
We hope this is helpful. Best of luck with your family research!