It wasn't like this in the morning it's just suddenly happend in evening. He has been eating fruits and veggies . Like The usual dietThe lighter more wet droppings are the ones that look abnormal, the consistency can be effected by what is being eaten, what is being eaten?
I think he chokedThere are multiple reasons why a bird can vomit, one of the common reasons is an infection of some sort in the crop. Did the vet do any tests, without testing it is not possible to determine the exact cause. I don't know what medications are available where you are but there are those that will help control the vomiting and then you need another to treat the infection if there is one, could be bacterial or fungal.