I have had the pleasure and privilage of being in the same situation. I have met with the auxiliarry bishop a handful of times. (the archbishop of seattle is still recovering from a total knee replacement and so the auxiliarries are doing all the day-to-day stuff, but Archbishop Brunette is still caling the shots from his bed so to speak)
I was very proper at first and used “your excellency” until he asked to be called Bishop Tyson, and I obliged. It was actually kind of nice, since it was a pretty casual setting and it did feel a bit awkward to be using a formal rather than a colloquial/familar title. I dont know how it is in canada, but here in the states we do have a tendancy to call church officials by their office. Archbishop N or Cardinal N rather than His Excellency or Eminence. but i would still advise out of courtesy to use the formal title until told otherwise.
Oh and I only use Bishop N after permission in casual or private settings. Formal settings or in public, if ever i run into the bishop (im in the cathedral city and i like to haunt the cathedral so it happens on rare occasions) i call him excellency.