If you have a purebred shepherd, then you will want to know how this breed differs from others. German shepherds are obligate carnivores, which means that they need a lot of meat to thrive. Even if you aren't a meat eater yourself, your dog needs most of his protein from animal sources.
The bare minimum is 22%, but I would suggest at least 45% of the protein in his diet comes from meat. Ideally, this will be complete proteins rather than animal by-products. Avoid fillers as much as possible.
German shepherds also have sensitive skin, so fat and fatty acids are an essential part of his diet. While too much can lead to intestinal problems, you want to make sure that he's getting enough to provide a thick, shiny coat.
This breed has an undercoat as well as a natural fur coat, so you have to make sure that they're eating enough to cover both. These dogs shed a lot during the year, so fat can help replace the hair more efficiently.
Moreover, you can talk to your veterinarians about your German Shepherd gets the exact nutritional needs and caloric requirements.