Jake was like that when I got him...all ribs and muscle. Couldn't seem to put weight on him, though I didn't try to feed him extra than I did the other dog. When I took him to the vet to be neutered I asked him about it and he said some dogs are just like that and they often grow out of it. And he was right! I fed Jake the same as I fed Lucy each day and he stayed ribby for a year or so and then started to slowly fill in and look just right. Muscles, skin and a little bit of cushion on the ribs now. Just takes patience, I guess.