Dear Ms. Mortensen,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located the series Enlistment Papers, 1798 - October 31, 1912 and Registers of Enlistments in the United States Army, 1798-1914 (M233) in the Records of the Adjutant General's Office, 1762-1917 (Record Group 94) that include military service files of the Regular Army. For information about the Enlistment Papers and M233, please contact the National Archives at Washington, DC - Textual Reference (RDT1) via email at email@example.com. For more information see Requesting Copies of Older (pre-WWI) Military Service Records.
M233 has been digitized and is available on Fold3 as Army Register of Enlistments, 1798-1914; on Ancestry as U.S. Army, Register of Enlistments, 1798-1914; and on FamilySearch as United States Registers of Enlistments in the U.S. Army, 1798-1914. 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.
We also located the Official Personnel Folders (OFPs), 1896-1951 in the Records of the Panama Canal (Record Group 185) that may include a folder for your grandfather if his position was as a civilian employee working with the U.S. Army. For information regarding these records, please email the National Archives at St. Louis (RL-SL) via email at firstname.lastname@example.org.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RDT1 and RL-SL. We apologize for this inconvenience and appreciate your understanding and patience.
Finally, the website U.S. Army South - A Brief History includes information about the Panama Canal / Caribbean Command that might be of interest to you, and the US Army Center of Military History’s online publication The Panama Canal: An Army's Enterprise may also be useful.
We hope this information is helpful. Best of luck with your family research!