I feed mine Welliness and Innovia. We have 2 schnauzers and a yorkie. Its important to feed them low fat food. Both breeds are prone to pancreatis. Also I just ordered mine the Nuvet vitamins that they say are really good to build their immune system and helps with allergies and all types of things. I hope they are as good as they say because it is suppose to even help with skin problems in schnauzers. We have a new baby and she is 5mo. old. My other schnauzer is 11. The yorkie is one. I am hoping they will all benefit from these vitamins.
I know that schnauzers have problems with allergies so I would go with a grain-free, single protein source food. Raw would be the best but if you're not comfortable with that, then maybe Wellness Core Ocean would be a good bet. It doesn't have a high fat content like other grain-free foods and there is one source of protein. I think they have a reduced fat formula too but dont' quote me on that. Also, try adding an omega supplement like wild salmon oil to his daily diet. That will keep his skin and coat in good shape.
In order to really enjoy a dog, one doesn't merely try to train him to be semi human. The point of it is to open oneself to the possibility of becoming partly a dog. ~Edward Hoagland