KMS- I hadn’t followed what had been going on with your pup, so had to go back and read your entire history.
Antibiotic usage/overusage can become a vicious circle. You might want to google antibiotic induced colitis. The clostridium overgrowth is one thing that can cause colitis from the toxins they produce, although it’s very possible there might be multiple causes.
When you said your pup has had full-blown diarrhea the last 5 days, that would very much concern me. Are you checking to make sure he isn’t becoming dangerously dehydrated? Lift the skin on the back of neck/shoulders. When released, the skin should spring back to to normal fairly quickly. Check the gums, they should be pink. Press on the gums briefly and pink color should return them within a second or 2. Mouth should not be overly dry nor saliva very sticky.
If the severe diarrhea isn’t causing an emergency situation, I would slow down on what you are introducing to him, with the exception of the pro-pectin you mentioned you already have. The main ingredient is kaolin, a form of bentonite clay, that should soothe and coat the intestines. It is also said to absorb toxins and bad bacteria. But you should also be aware that kaolin will interfere with absorption of some antibiotics, and specifically trimethoprin (the TMP in the antibiotic combo you mentioned). Pro-pectin also has one form of encapsulated beneficial bacterium in it. Then I would wait for the outside lab results, and then maybe go for the colonoscopy if advised.
If I were you, I wouldn’t yet even think about adding raw meat to his diet until you can get his intestinal flora back in balance, although a homemade lightly cooked one could be the way to go for awhile….
