Hello and welcome. You might start by gently looking in his mouth with a flashlight by pulling on his wattles and very gently prying his upper beak. ( This is usually a two person job) Look for anything unusual, or whitish or yellow lesions. Be very careful as the beak can break easily especially if the bird struggles. If it all looks normal, then try starting him on an antibiotic like Oxytet (2 tsp per gallon) if you can still find it in feed stores, or Gallimycin. Only saying this because roosters seem to be prone to getting this weird voice thing where their crow is squeaky and sometimes dissapears altogether. We haven't had any cases of it for years, but we have had a couple, and that is how we treated it. It seemed to clear up on antibiotics, with no issues, but if you put him on antibiotics, he does need to be on them for seven to ten days, and so he should be in a pen by himself, which also protects the rest of the flock. You can also try injectible Tylan, which doesn't require such a long course of treatment, and if you have other roosters, that would be the best way to go, because he will have a hard time regaining his status in the pecking order if he is confined away from the flock which contains other roosters. If you can't get the antibiotics (and you won't be able to at all without a vet's prescription after the first of the year)..then a vet may be the only way to go. If nothing else, you can always try VetRX, but I have never had very good luck with that stuff. Some people swear by it though. You can find it on line or in feed stores.
If there ARE lesions in the mouth, than that is canker and needs to be treated with Metronidazole, which you would probably need scrip from a vet for.
PS: I just noticed you are in Canada, so that might change availability of antibiotics for you. Also, Gapeworm is not very common in chickens here in Ohio, but I don't know about Canada. I also don't know if Gapeworm would show up in a fecal float.
