Hi again, Anna/Justin/Jaime!
I think you need to pool your resources and look at everything that's on his marriage/death certificates.
For example, when exactly did he marry and can you look at all the info there (e.g. witnesses to the marriage).
You say there's nothing on the marriage certificate apart from his birthplace - is there a space on the marriage certificate you have for parental info and it's been left blank?
On the death certificate it may say something about how long he's been in Australia.
You could also check with the library local to where he married for newspaper records - it might say "nephew of" somebody local.
The death certificate might not be the best source of info as his family in Australia might not know the full details.
http://cap.estevan.sk.ca/library/ - Estevan isn't that large a place. They do have some cemetery records and other local records up on the library website.
I'm simply not having any luck finding Stephen or Ruth in Canadian records. If they died in the 1920s/1930s that might be difficult to find, though. As Estevan is close to the border with ND they might also be hiding there (or even further afield; Montana?)