Hmm, because the shape of the discolored area seems the same regardless what color he is, it suggests some sort of skin problem in that spot. The location is a common one for a burn. A bruise would stay dark constantly for a few days then disappear. The skin texture would look normal with no swelling, crustiness, etc.
I think just to be safe I'd dab some Polysporin on the discolored spot and keep an eye on it for changes. If the edges of the patch start to look crusty, pink, or raw, it might be a burn. If nothing changes and the patch goes away, the Polysporin wouldn't have hurt anything. If the patch spreads, take him to a vet as it could be some other type of infection such as a fungus. Antibiotic topicals like Polysporin won't treat that.