Switch the dog to a species-appropriate, low carbohydrate diet. A raw, home cooked, dehydrated or canned diet free of any grains or starches is ideal. If you must feed kibble look for a high protein (>30%) grain-free/white potato free kibble. If kibble is necessary, I think somebodysme made a good suggestion with Nature’s Variety Instinct.
I would also highly recommend starting the dog on a high quality, multi-strain probiotic supplement and an enzyme supplement. Probiotics are “friendly” bacteria that will help to get the body’s microflora back in balance (a yeast infection is an imbalance of microflora). Enzymes will help to rid the body of toxins that can contribute to yeast overgrowth and help the dog to more efficiently digest the food.
Yeast infections can also be caused by deficient levels of IgA and German Shepherds are prone to IgA deficiencies. If there is an IgA deficiency the probiotics and enzymes should help and you may also wish to consider an immunoglobulin supplement (such as IgG 2000 DF or colostrum) and coconut oil (the lauric acid in coconut oil is beneficial to immune function). You vet could test for an IgA deficiency.
You should also regularly disinfect the parts of his body that are yeasty. You can use a solution containing 1 gallon water, 1 C. hydrogen peroxide and 1 C. white vinegar. You can also bathe him using an herbal shampoo (such as tea tree) – avoid oatmeal shampoos as oatmeal feeds the yeast.
Good luck! 🙂
