I agree with Bullchick... birds hide distress well, so try to observe him when he is stress free and preferably off on his own where he doesn't have to keep up appearances. If it is to his side it can be a torn muscle with swelling, a cyst, an abscessed wound, even a fractured rib pressing against the skin with associated swelling... almost anything. And yes, an animal can 'dislocate' their spine. It isn't common, and usually causes a lot of neuro symptoms but vertabrae can be 'subluxed'.... basically an incomplete dislocation... which can be realigned with chiropractic manipulation in some animals (I had this happen with a dog) but I can't imagine how it would work on a chicken.
I also agree that breeding may be out, unless it is found to be an injury, and depending on observation findings culling may be needed.
I also agree that breeding may be out, unless it is found to be an injury, and depending on observation findings culling may be needed.