Hello there
If the crop feels hard and unchanged from the previous day, it is most likely impacted.
Sour crop - soft and squishy crop in the morning. Will have s fermented smell. Caused essentially by the crop not functioning properly.
Impacted crop - as a result of food gorging, eating long fiberous materials or substances more difficult to digest, the crop is not clearing and the food has compacted into a ball, not allowing it to clear.
After a bit of time, the food can begin to ferment in an impacted crop, which would lead to an impacted, sour crop.
To relieve the impaction, you can drip an olive oil into his mouth, and massage the crop. What you are trying to do is break up the material in there and aid in his digestion.
It is very important to massage the crop down toward the ground, or his stomach.
Chickens are not designed to throw up, or have anything come back out of their mouth. It can easily cause them to choke.
Also, the olive oil. We are talking small drops. We don't want to blast a syringe full into the guys mouth. The thick consistency of the oil can easily lead to aspiration. Just take your time, small drops and be very gentle with him.