Citizen's don't have to be living in Canada to sponsor. Instead, what I would do is send them a copy of his work permit (which should have an expiration date on it) as well as other return-to-Canada information (e.g., family/friends with whom you can stay when you return, etc.) They definitely should not be asking for evidence he is in Canada now, not as a Citizen. I'd send them a copy of his passport as well and a nice cover letter that says "I'm a Canadian citizen, living abroad with my spouse. Once you approve the visa we plan on moving to Canada. Enclosed is evidence that I plan to return by X time, but given how long this process has already taken, I reserve the right to change this should you be unable or unwilling to deliver my spouse's visa by that time."
I'd definitely send them something - not answering gives them an excuse to refuse. But answering with something, even if it is not what they request, means they have to follow-up with you.
