My case isnt exactly the same as yours, but wish you can take it as ref. I have 2 GSDs from different rescue groups. Female is now close to 7 (est), and the male is 3 months to 3. When i got the boy, he was only 11 weeks. They rescued the parents, and the mom gave birth 3 puppies in foster home. I got one of them. He had been very health until around 18 mo old. At one point, he was 78lbs, but dropped 10lbs in 1 mos while eating 5 cups a day. Foods wasnt bad, i fed Origen, ~US$100 per 28lbs bag. Why all the sudden with the same food? No food recall, no formula changed. Why? I tried few other brands, Hills (suggested by vet), TOW, Zignature LID...etc. no luck. He just keep eating a lot with losing weight. But he was still very playful as puppy. Then on luck, i met another GSD owner at dog park, and i was told about EPI (Search it yourself if interested). Another vet visit with $400 blood test. Bingo. And lucky enough, he is on the light side. Then of course another expensive bills and med costed me like $3000-$4000. And the med didnt work at all.

So, after closed to 10 mos of frustration, i tried raw diet (Primal) just for a kick. And it worked like magic. Poop turned back normal in just 2 days. Vet suggested that there are some dogs that cant take any kibbles. I was suggested to keep raw feed fir 2 mos before trying anything else. So, in the last 2 months of 2014, he enjoyed 3lbs of meat (chicken or beef and liver, all raw) a day. He gained 8lbs back before the new year. Now, he is on The Honest Kitchen as main plus some raw once a week. Poop not prefect as 100% raw but still good. And weight keeps going up now just over 80lbs.

My suggestion is that, give him raw feeds for a try. 2.5~3% of the body weight daily as guideline. You should see the differences in short time. Dont mix raw with kibble. Just switch 100% in once. Do some research before you try it out. I know some vets against raw diet, but it works on my boy like magic. Oh, he also had some red hotspots, but now all gone.

Good luck with your dog.