I would guess that Stewart Harris of Cedarville, Ohio, in 1920, received that name in Ohio. This is his census information from 1920:
1920 Cedarville, Greene County, Ohio
Elmer Spencer, 46, married, born in Ohio, parents also, farmer, general farm
Mary F Spencer, 69?, married, born in Kentucky, parents also
Charles M Harris, 46, stepson, married, born in Ohio, father in Ohio, mother in Kentucky, farm laborer
Myrtle S Harris, 42, stepdaughter, married, born in Ohio, parents also
Stewart Harris, 7, stepgrandson, single, born in Ohio, parents also
Myrtle [something], 2y or 2m, lodger, born in Ohio, parents also
https://www.familysearch.org/ark:/61903/1:1:MDYJ-P4M?lang=enCharles and Myrtle were brother and sister, not husband and wife. Their mother Mary was previously married to John S Harris, a marshall in Cedarville who died in the line of duty. John S Harris' grandfather was named Stewart Harris. [Was John's middle name Stewart?] There is a Stewart Cemetery in the area. Here is a brief tree, based on the tree at familysearch, and a few findagrave sites. I didn't verify the information but it seems to hang together pretty well.
Evan Harris (1755 - 1836)
+ Rachel Stewart (1757 - 1838)
- Stewart Harris (1785 - 9 Jan 1869)
buried Stewart Cemetery, Bowersville, Greene County, OH
https://www.findagrave.com/memorial/38574186/stewart_harris + Mary (1784 - 1852)
- Evan Harris (1805 - 1867)
+ Phoebe Smith
- William Harris
buried Stewart Cemetery, Bowersville, Greene County, OH
+ Elizabeth
- John S Harris, marshall (1849 - 1883)
+ Mary F Albright (c 1849 - )
her second: Elmer Spencer
- Charles M Harris (c 1874 - )
+ Belle Emma Cline
- Maude M Harris (1895 )
- Ilo B Harris (1897? )
- Stewart Harris (1912 - )
- Myrtle S Harris (c 1878 - )
Charles M Harris in familysearch tree
https://www.familysearch.org/en/tree/person/details/L4B3-RXQhis wife, Belle Emma Cline, remarried in 1914. Daughter Maude is with Belle in 1920. Daughter Ilo is married by 1920, leaving Stewart with his father and grandmother.
Charles, Belle and daughters in Cedarville in 1910
Charles Harris, 37, - owner and manager of a restaurant
https://www.familysearch.org/ark:/61903/1:1:MLX3-MMR?lang=enJohn S Harris at findagrave, but without his second wife, Mary F Albright
https://www.findagrave.com/memorial/21547143/john_s_harriscomment on greene county site discussing grandparents and brother of John S Harris:
"Within the family the last known residence of William and Elizabeth was on Dayton Hill in Xenia. William was the son of Evan Harris, who was first school teacher in Caesarcreek Twp. Evan was the son of Stuart Harris an early settler. Williams brother Marshall John S. Harris was the first lawman killed in Greene County (Cedarville). Both William and Elizabeth are buried in Stewart Cemetery, Bowersville. My grandmother Edythe Harris Davis was raised by William and Elizabeth at the residence listed in the dispute."
https://www.greenecountyohio.gov/Blog.aspx?IID=277Which all means that the child born to Janet McCallum in Glasgow in 1912 most likely did not leave Scotland with the name Stewart Harris. Or Canada for that matter, if he was ever in Canada.
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Edited to add: this family has no apparent connection to Scotland for generations as far as I can see, so I don't think they received Stewart directly from there. I might be wrong though. My guess is they were looking for a farmhand and got Stewart somehow, legally or otherwise.