The problem is that sores are caused by everything from ringworm, to flea allgeries, to cancer, to FIV and FeLV, to fights... the list just goes on and on. Only a vet can tell you for sure what's going on, and he HAS to go. HAS TO. It could be signs of something severe, that can either be curable, contagious to your other two, or might make him suffer a slow death.
I'm not sure if I can post this, but in this area I could give a ____. Go to www.thecatsite.com/forums/ and ask there what kinds of programs are available in Canada for low-cost vet care. I can tell you how to do it the US, but I know that Canada has much different programs. England is the only country I know of that has a great system, other than a handful of large US cities. There SHOULD be a low-cost clinic in your area, and if that fails, start calling the humane societues, shelters, and rescues. There are tons of rich people who volunteer at these places, and sometimes they'll go out of their way to help.
Good luck!
any way im not being nasty at all just feel the need to tell you that there are places out there for those who dont have loads of money (myself included as it all gos on cat and fish food 
) we have the blue cross in England you will have your equivelent over your way.
i also would de flea them all comb them through with a flea comb to get the buggers out as oftern as it takes to get shut of them it could also be a reaction to the bites he may be skinny because hes unwell as the fleas suck all the goodness out of him and he may be aneamic look out for white or receeding gums pale eyes the third eye comming accross and runny eyes and nose sneezing coughing , shaking of the head, walking low ( looking like he has belly ach kind of walk) and unkept fur these are all signs that a cat needs more help than you may be able to give, some vets let you have a tab where you can get treatment and pay in instalments, ive had to doit many times our Poppy thinks she can fly and has broken more bones than i can name and paying for operations for them to be reset or amputated is a big effort i know .How old is the cat?
