Put my dog in a diet, now he's too skinny.?

My miniature pinscher has degenerative disc disease and an enlarged heart. His vet said that he was over weight and that he needed to lose some weight to keep from being in pain. I used to keep his food and water dish out all the time, but after talking with the vet, I began only feeding him a cup of food once a day at the vets recommendations. The diet worked and he lost a lot of weight and isnt in pain anymore. In fact he is actually thriving and is so much more acrive. So I began feeding him twice a day, but he just keeps losing weight. Now he is just so skinny that he almost looks sickly. He acts perfectly fine. He still runs and play, he us always loving and is always alert. He barks all the time!!! But he is just beginning to look so thin and its worrying me. I keep feeding him, but the weight keeps just falling off. He doesnt have worms or fleas, and other than being too skinny, he is perfectly healthy. How do I put more weight on him without causing more stress to his spine and heart?