Yes it is a chance he is is not listed. These cemeteries were recorded from Headstones, if family did not pay for a stone, it's not there.
Did you have a good look at that AGS site. He is on the Book Index there. And they will send you the page he is on.
Some of these cemeteries may include the records may want to ask the AGS if they could have a look at the Westlock Cemetery File.