Obtaining PR and actually moving there are 2 different things. Some make the move, others do not. He did not leave any of his posts or resign to make the move unlike his family. I am dead certain that you are not liable for taxes in Canada even if you are granted PR until you actually move there.
You mentioned that the entire family obtained PR back then. If his wife and kids had remained in Canada, he should have filed taxes for 5 years (period before his PR lapses).
Did he? Anyway this is all speculative. But I am sure there is a lot more to the story.