Is mixing the dog food with Purina cat food ok for my GSD?

Ok, for the past week, my dog hasn't been eating. He's so skinny now to the point his rib cage and spine is visible. Some people advice me to take him to the vet, in case he might have stomach ulcer or something worse. Others say I should change his food brand, cos maybe he's just too bored with his food. I can understand how he feels cos if I were to eat cereal for every meal for one year, I'd be bored too! I realised he loved my cats' food, cos occionally, he would steal it. So I mix his food with the Purina cat food (dry) this morning, and he was just eating like a hungry pig! In a matter of seconds, the dish was all empty. I tried it again in the evening, and again, he chows it all down. My husband say though he's relieved that the dog's eating again, the cat food may not be suitable and I should find alternatives. I'm worried too cos GSD are known for their delicate health. Is it really bad to mix the cat food together w/ his food? What should I do?