Canker is a fungal disease and as Jean said, highly contagious. It is a carrier disease as well, I've read (though that seemed surprising to me), meaning it will keep popping up in the flock and is never truly gone. I'm not 100% sure, but that does seem to be canker. You'd have to have him tested to be sure.
We once had a well loved rooster and discovered he had canker. Had no idea what to do and my dad adored him, too (he was my sons roo) so he went to the feed store and asked them. That's what they said. Brought it home, administered it. It did work. Google shows a lot of people do this as well.
It's worth a shot. It's not cheap, but it's not super duper expensive, either.
The Flagyl will work good but also it should get a dose
of antibiotics. Search for Doxy-T. Works real good
for canker. Canker can be fungal or bacterial
and spreads mainly through the water supply when the
birds drink or can be transmitted through saliva from one bird to
another bird.
It's more common in pigeons than in poultry but can be treated the same way.