sounds like it may be allergies.. i would try switching first to a grain free food and if that doesn't work, to a different meat source (ie, he may be allergic to chicken). GSDs do have a very sensitive stomach and our guy did not do too well on any commercial dog food, we had to switch to raw and glad we did so.
Monkey and Amy (cats)
“Dogs believe they are human. Cats believe they are God.”
"The average dog is a nicer person than the average person."