likes biting his hindquarters. I think he may be kind of itchy and dried out
That is allergy. Any puppy would gobble whatever you feed him. GSD is a breed must be raised on raw - that is the only way to raise a healthy dog. And better - organic. He needs yogurts, cottage cheese, fish, raw chicken, raw rabbit and beef mince 100 gramms twice a week - all necessary vitamins and minerals the carnivour gets from fresh raw meat. Your dog's young organism would struggle until he's really sick at 7-9 months, than the system will fail. Would you feed your baby dry bisquits as well ? Stop experimenting.