He has to notify them BEFORE he lands - as soon as he notifies them his PR visa will be stopped and he will need to fill out new paperwork to add you to the application. So I don't think there's any reason to wait - the sooner he gets the ball rolling, the sooner this will be finalized for both of you. It doesn't matter that you're already in Canada - if he does not add you to his application, you will be barred from sponsorship later on. They're very strict about this - read through the appeals cases by searching this
CANLII website for 117(9)(d). It's not a pretty sight.