My girl got it from eating clumps of grass i stupidly gave her. basically, they eat the food, it moves into the crop where it digests before moving into the stomach. If they eat something, like straw, random trash, or in my case a clump of grass, it may get bound up like a nest and block the crop from emptying. The bird then continues to eat piling the new food on top of the "nest" so then that food begins to ferment and bacteria, yeast etc. begid to grow making the crop squishy. make sense?
It may not be anything, he may be fine, but just to be sure, what I would do is remove the food tonight (which it looks like you already do-btw chickens don't eat or drink at night so its totally ok)and then feel his crop in the morning before you give anyone food. If it is still squishy come back to BYC and look up sour crop - or shoot me a message, I'll let you know what I did with my girl.
