He is married to Mary Thomson.
They got married on September 8, 1873 at Beith, Ayrshire, Scotland .Sources 1, 2
https://pigott-gorrie.blogspot.com/2017/11/robert-young-father-and-son-of-ayrshire.html
Record for William Calderwood & Mary Thomson/ Ancestry.com
Name: William Calderwood
Gender: Male
Marriage Date: 08 Sep 1873
Marriage Place: Beith, Ayr, Scotland
Spouse: Mary Thomson
FHL Film Number: 6035516