Thank you!You'll have to open the rooster's mouth, inspect the roof of his mouth and the back and sides of the mouth as well as the entrance to the esophagus at the back of his mouth for tiny white splotches. The white splotches ARE the yeast infection.
In conjunction there might be mucus in the esophagus causing him to gurgle when he crows. It's a good indicator that the yeast infection is in the crop as well.
Where did you buy the acidified copper sulfate?