It sounds like you are throwing all kinds of foods at this pup. Digestive upset can cause bloody stools. I would take him back to the vet to figure it out (what if it is worms again?) and consider settling on one food that agrees with him for awhile. GSDs can have sensitive systems.
NAPWDA Certified Cadaver Dog
Waiting at the Bridge (italics
=GSDs) (hemangiosarcoma=blue):Grim , Cyra, Toby
, Rainbow, Linus, Oscar
, Arlo & Waggles