The copper sulfate won't hurt anything to give in the water. It's used all over the world in livestock tanks as a common practice to prevent algae and parasites that can cause the sort of problem your rooster has.
It probably hurts to swallow and that's why he's lethargic, being weak from hunger. Try putting a little sugar in his water along with the copper solution. The sugar can raise his glucose and he may feel more like eating and moving around.
Sometimes canker gets so bad that the chicken can't breathe so this is fatal in some cases. Keep up with the other meds the vet gave you. They're no doubt keeping the canker from getting that serious.
Good luck. I hope the copper tips the scales and he recovers quickly.
It probably hurts to swallow and that's why he's lethargic, being weak from hunger. Try putting a little sugar in his water along with the copper solution. The sugar can raise his glucose and he may feel more like eating and moving around.
Sometimes canker gets so bad that the chicken can't breathe so this is fatal in some cases. Keep up with the other meds the vet gave you. They're no doubt keeping the canker from getting that serious.
Good luck. I hope the copper tips the scales and he recovers quickly.