Martha, According what I read in this article, depending on how old the dog is GS dogs need 22% protein and 8% fat until they are 6 months old. Adults 18% protein and 5% add 5% fat . It also gives a lot of other information about feeding. Did the vet mention EPI,(see below) if not you might try the suggestions they recommend in this article and see if he responds. Do you know what they have been feeding him? It could be that the food doesn't have whole meat protein as the main protein ingredient as suggested below, or it has BHT or another of the synthetic preservatives in it. If the vet is suggesting one of the expensive foods that they sell, don't feel pressured into buying it as many are filled with the ingredients below like the synthetic preservatives. IMHO vets and doctors are not taught good nutrition, they may know the numbers but they don't consider the additives. If you let me know what exactly the vet recommended and you want to buy a more natural food let me know.
Also I would talk to your vet again when you have read more and when you have found a food you think would fit his needs take the list of ingredients with you to show him.
Natural preservatives such as vitamin E or vitamin C may be preferable to synthetic food preservatives such as butylated hydroxyanisole, butylated hydroxytoluene and ethoxyquin. These synthetic preservatives are controversial and could cause health issues, according to the FDA.
http://www.petcarerx.com/article/health ... epherd/506Read especially what it says about bloat as they are one of the breeds that are prone to it.This is what it says about weight Loss
Some German shepherd dogs may be at risk of conditions that lead to the improper absorption of nutrients from their food. These conditions include exocrine pancreatic insufficiency (EPI) and small-intestine disease, both of which can cause weight loss, even as your dog shows an increased appetite, according to The Merck Veterinary Manual. If you notice that your German shepherd has lost weight but appears to be eating normally, consult with your veterinarian to see if a health condition such as EPI, small-intestine disease, or some other health issue could be the cause.
Part of the treatment for these conditions is a diet that is low in fiber and fat, and that contains a high-quality, unusual protein such as venison or lamb as the primary ingredient. EPI or small-intestine disease diets also contain highly digestible carbohydrates, such as rice or potato. Such diets should be overseen by your veterinarian, who will monitor the results and determine whether they are effective for your German shepherd dog.