Dear Ms. Shaw,
Thank you for posting your request on History Hub!
Many Canadians served on both sides of the American Civil War. There is no one state whose forces they served with. For those that served the Union, it would be logical to focus on the American states closest to their place of residence, but that does not necessarily mean that they must have enlisted there.
We searched the National Archives Catalog and located the series Carded Records Showing Military Service of Soldiers Who Fought in Volunteer Organizations During the American Civil War in the Records of the Adjutant General's Office, 1762 - 1917 (Record Group 94) that include the service records persons who fought in Union volunteer units during the Civil War. Some of these records have been digitized and are available using the Catalog. This series is indexed by the series Indexes to the Carded Records of Soldiers Who Served in Volunteer Organizations During the Civil War, 1890-1912 which has also been partially digitized.
We also located the series Enlistment Papers, 1798 - October 31, 1912 and Registers of Enlistments in the United States Army, 1798 - 1914 in Record Group 94 that include military service files of enlisted soldiers of the Regular U.S. Army. These records have not been digitized. In addition, we located Letters Received, 1863 - 1917 (documenting 1850 - 1917) in Record Group 94 that includes letters, reports, and papers relating to the military service of officers of the Regular U.S. Army, officers of Volunteers in the Staff Corps commissioned by the President, and officers commissioned by brevet appointment in the Regular Army and Volunteer organizations. Portions of these series have been digitized and are available using the Catalog.
Plus, there are pension records for some Union soldiers and their survivors, the majority of which have not yet been digitized.
Lastly, we located the Carded Records Showing Military Service of Soldiers Who Fought in Confederate Organizations in the War Department Collection of Confederate Records (Record Group 109). Many of these files have been digitized and are available using the Catalog.
To order records that have not already been digitized, please fill out a NATF Form 86 for the service records and a NATF Form 85 for the pension records and mail the completed forms to the address listed on the form. You also may place your order online using eservices.archives.gov. For more information see Requesting Copies of Older (pre-WWI) Military Service Records.
Please note that the National Archives and Records Administration (NARA) has suspended reproduction and digitization services until further notice due to COVID-19. Orders will not be serviced until operations can resume safely. Once operations resume, document reproduction requests will be filled in the order in which they were received. We apologize for any inconvenience.
The National Park Service’s Civil War Soldiers and Sailors System (CWSS) is a database containing information about the men who served in the Union and Confederate armies and navies during the Civil War.
If he served in a state volunteer unit and you can identify which state, the state archives for that state may have additional records about his service.
We also searched online and located the following articles on the topic of Canadian service in the American Civil War.
We hope this is helpful. Best of luck with your family research!