He was actually listed on the 1921 English census, rather than the Scottish one, so if you go onto Findmypast and search the 1921 census for Douglas Holland born 1899, he will come up and it will show you his rank. You don't need a subscription to view the image as they do pay per view, but you will need to make an account if you don't already have one. Then when you are viewing the image, click where it says Extra - Address and it will show you the name of the regiment (also shown on the original image) and where he was.
https://www.findmypast.co.uk/1921-census