It could be his diet, and it could be sunburned feathers. In either case neither vinegar nor bluing will remove the stain or burn--the only thing that will do that is a molt and nedw feathers.
Vinegar will get out some of the stain, depending on the extent to which it has integrated into the feathers, and it will help get out any minute bits of dirt clinging to the stained areas.
Bluing applies an optical illusion that makes the feathers look brighter, but to all extents and purposes, it is a dye, just a very pale one when properly applied.
Blu-cote contains gentian violet (hexamethyl pararosaniline chloride), which is an anti-microbial. As a side effect/benefit, gentian violet is a very strong dye that covers the red of blood or raw skin, which can attract the predatory nature of chickens.
Bluing contains ferric hexacyanoferrate, a synthetic dye named "Prussian Blue."