I'm assuming you have an inland application in progress?
There are two types of interviews for inland apps:
The first is the one that might signal trouble and is usually caused by the IO not being certain that the relationship is genuine. Or sometimes it's just because you didn't supply enough information with the application. This one is more of an exception than a rule, most applicants don't need it. It's not the end of the world, just means a bit of a delay.
The second is the 'landing interview'. Every applicant has to do this one - it's when they become a PR. It's just a formality really, they only ask if everything on the forms is correct, if the marital status is still the same, if he declared all of his children and if he's been convicted of a crime recently. As I said, just a formality and definitely a good thing as it's the end of the application process.
Since you got first stage approval and he has an open work permit, I am guessing he's being asked to come land. In which case, congratulations!