My German Shepherd has become way too thin!!?

I adopted one of my Gsd's from a lady who could no longer take care of him due to her cancer. When I got him he was a healthy weight and filled out nicely. I've had him for 4 months now and he's gotten so thin! I can see some of his backbone, ribs and hip bones. It's not extremely bad but enough so where you can see them! He's been to the vet and current with all his shots. I feed him about 6 cups a day with diamond naturals once with an egg in his food and some all natural dog food roll. It's basically lime a beef jerky roll but made for dogs. He's also an inside dog and gets one hour a day of outside play off leash. All he does is chase his toy so he gets exercise but not as if be were an outside dog. I also have another inside shepherd who has perfectly normal weight. So what gives? The vet says he's fine but he's just too thin! Should I feed him more or switch to another premium food? I feel like if the ASPCA saw him they would take him away because it looks like I don't feed him. Any suggestions anyone??