The apple cider vinegar you use needs to be the natural ACV with the cloudy "mother" in it. It will have more of the good bacteria you need for your birds. The processed ACV removed the "mother". A good variety is Braggs but any ACV that says "natural" or "mother" on it is okay.
Also, if you attempt to empty the crop, be careful that the bird is allowed to breathe right side up. You don't want him to aspirate the vomit into his lungs which would cause a whole other problem.
Crop is the first receptacle in the digestion process. What the bird eats is mixed with digestive juices and water to begin the softening process before it moves to the proventriculus( or true stomach) where enzymes break the food down further. The food is then passed into the gizzard (or mechanical stomach) where the food is ground using small stones and grit befor moving into the intestines.
You described your bird as hiding his head between the 2x4s. Very possibly, he is wolfing down his food when he can get to the feeder and is stressed by fear of the bigger birds. This can also contribute to a sour crop.