You could try posting on these websites.
http://www.animalrescuenetwork.org/
http://www.canadastrays.com/default.asp
http://www.rapt-lfpn.org/en/accueil.html
http://content.petfinder.com/classif...sifiedHop.html
http://www.flealess.org/lostpets/
Are you visiting the humane societies, or simply calling? I recommend that you go there every few days instead of phoning, shelter people are always so busy and your cat may be overlooked - unless he's microchipped or tatooed or still wearing his tag.
Don't assume he's gone any great distance, particularly since he's an indoor cat and won't have many street smarts. He'll be doing a lot of hiding. If your neighbours have sheds or garages, ask them to check that he isn't trapped inside (this happened to one of my cats; luckily, he managed to escape after a week).
You could fax his picture to all the vets in your area as well, in case he is brought in.
There's still hope, don't give up.