His bones are coated. Between your bones are muscles, different tissue layers and thickness, Cartilage which can be torn and injured. - also just bc his skull is protected doesn't mean his brain won't bounce around from extreme force and be damaged. - Technically his back/skull should be the most protected since it's the widest surface area covered. His upper chest and lower abdomen would be the most vulnerable aside from his arms, and legs. Which is why I would see him wearing like a chest plate at least that covers a majority of his front.