Does molting happen only in the autumn? I'm asking because none of my chickens went through it & now my rooster is suddenly losing feathers underneath his wings, tail & on his chest/underside. I didn't find any signs of mites (still going to treat them/the pen & coop for it), so I'm at a loss as to other reasons why he's losing feathers there. He's also the only one losing feathers. The hens are in great condition.
Have a good look over him, checking his skin at the neck, under the wings and his vent. Look for bugs, or anything that might be causing him to loose the feathers. If you're feeding good food, and he doesn't have bugs, I'd say he's molting. Just keep an eye on him, and get him some extra protein, which will help if he is molting. Eggs, BOSS and calf manna are great for adding protein.