Whenever I take Tony to the beach I always get a can of dog food pumpkin or a can of 100% pumpkin with nothing added. That really helps with diarrhea and Tony always gets diarrhea after going to the beach. I would first have him checked out by a vet, who knows GSD's. He could be allergic to something, I would try a grainfree and one without chicken. Those can be issue's with our dogs. Also many GSD's are allergic to flea's and even one can give my dog fits. Sand flea's can also cause issues with our breed. Have him checked by a vet and take a stool sample with you, the vet will need that.