To me it sounds like he's not getting enough to eat, 3 cups of purina a day is not enough for a dog his size, I don't beleive. Since you are changing to be a better quality food then 3 cups a day imight be enough, but many dogs eat 4+ cups of purina types foods a day just to try to maintain weight, as it's mostly full of fillers. What food did they recommend to you on the GSD board?