The border officers now know about the "flagpoling" option. It's up to you. Your father could either send in an extension application or exit Canada and come back. To exit to the US one must have a valid US visa though.
So what about other options, I mean I have heard that ppl just go to the US border and get a new entry ... leaving canada and then coming back in ... but for that dont u need a valid US visa as well. Please advise