Thrush can be found in the crop, gizzard and possibly on further down, including canker. I've dealt with yeast infections in my old Buff Leghorn rooster. It doesnt permanently go away and requires continued treatments over time.
Other symptoms you might see are gurgling when he crows, difficulty swallowing feed, mucus/saliva in the mouth and in esophagus, drinking water excessively.
Antibiotics shouldnt be used while treating fungal issues unless it is absolutely necessary.
I recommend using Acidified Copper Sulfate. It will clear it up until you see symptoms again which varies with your environment. Here where I live in Florida, I have to treat my rooster once every 4 or 5 months with the Acidified Copper Sulfate (ACS.) We have a warm/mostly wet environment.
In between times I use an Epsom Salts solution albeit a temporary fix with less treatment time of 3 days.
Here's where you can order ACS:
https://www.cacklehatchery.com/product/acidified-copper-sulfate/
Here's the instructions on how to mix and use ACS. See post #4 in this link:
https://www.backyardchickens.com/threads/canker-or-fowlpox.1648534/#post-28263234