What other symptoms does he have that lead you to believe he has sour crop? Usually, the yeast infection won't permit digestion of food, and it sits in the crop along with a lot of liquid, making it feel like a water balloon. What has his poops been looking like? Are they thin and watery and greenish? Or are they tiny, scant and hard? When you feel his crop, does it feel like a hard, small mass in there, or is it completely empty?