From the CRA point of view, he was unemployed and so he does not necessarily have to file it right? What I'm thinking is, can he just proceed without even filing 2017?
There are many reasons why a resident of Canada SHOULD file a tax return EVEN WHEN the individual is NOT REQUIRED (by CRA rules) to file a return.
But that should not be confused with what is required to qualify for a grant of citizenship.
What is required to be eligible for a grant of citizenship is COMPLIANCE with CRA tax filing rules for at least THREE of the five tax years preceding the tax year in which the application is made.
This means that for any tax year in which the individual was NOT required (by CRA rules) to file a return, that year COUNTS toward the three needed even if the individual did NOT FILE a return for that year.
Thus, yes, an individual could, for example, meet the citizenship requirement to comply with tax filing rules without ever filing any tax returns, if for at least three of the preceding five years the individual was not required to file a return (according to CRA rules).
That said, the REQUIREMENTS are one thing, BUT what might raise questions, and cause problems, can be quite another.
The prudent PR will file a return every year, and even though filing compliance is only required for three of the five years, actual compliance for ALL years is the much, much better approach.
This is not to suggest going back and filing a return for a previous year in the past and for which the individual is not required (by CRA rules) to file a return. That is for the individual to decide, but I believe benefits which may have been available, upon filing a return for such a year, are probably no longer available. Not much to be gained by filing an old return now. And as for the impression it makes on citizenship processing officials, the employment history will tell that tale anyway.