I would look at airlines like BA (they have a Skyflyer Solo service) and Air Canada as they may be able to assist. Finding him a direct flight to Ottawa is not going to be easy, such flights I think actually only depart to there from Heathrow.
I put an unaccompanied minor on a Virgin Atlantis flight to Chicago recently. The service was great. They were much cheaper than BA. I recommend them. Although don't know if they fly the route you are looking for.
I was very nervous. DD was not. :-(
Got an email fromher today where she said "stop being so fussy mummy".
Have you checked air transat. We flew with them recently Manchester to Vancouver. There were 2 young kids flying unaccompanied looked like brother and sister, on the flight out. I would say they looked about 12 and 10 years old. Hope this helps.