DeeDee, Thanks for your question!
GSDs are the poster children for disease like EPI (exocrine pancreas insufficiency) and are also prone to chronic inflammatory bowel disease, so talk to your vet specifically about those diseases. He's a little old to be diagnosed with EPI but still worth a conversation and more diagnostics eg and abdominal ultrasound.
GSDs are also prone to other immune mediated skin diseases like lupus and pemphigus so I would potentially consider seeking a consult with a veterinary dermatologist.
