HELP!!!!! Fatty Tumors in Stool??!

[Samantha Fine]

Steve is quite the mystery pup. He is almost 6 months old. Recently (two weeks ago) we left him with a dog sitter who had two dogs and a cat. When we got him home he was very quiet and distant...not himself and drinking a ton of water. Fast forward 4 days later, he passed these fatty/fleshy tumor looking things in his stool and came home with fleas GRRRRRR. The vet is at a loss as to what it could be, they tested his fecal for parasites and they came back negative. He wanted to treat Steve as if he passed a foreign body and see how things progressed. He was put on Anti- Diarrhea and Intestinal Antibiotics. They seemed to help a little but his stool was borderline diarrhea and solid. He has really solid stool in the morning but seems to go to water throughout the day. He is still very energetic, eating and drinking regular, no vomiting, nothing seems out of the ordinary until he poops. Has anyone seen this? Recommendations as to what to do? 

[F, Calla & Luca]

If your dog is missing the enzyme lipase perhaps this is just undigested fatty material he somehow ingested. Mjust me best guess.