You may also find it helpful to consult a Naturopath or Herbalist or Homeopath(not necessarily the same as a Holistic Vet) https://natureswaypets.com/links-book-list/

SLIPPERY ELM
In the gastro-intestinal tract, Slippery Elm acts directly. It can be thought of as a sort of natural “Pepto-Bismol.” (Pepto-Bismol itself should not be used because it contains salicylate, a.k.a. aspirin). Its mucilage content coats, soothes, and lubricates the mucus membranes lining the digestive tract. Slippery Elm is an excellent treatment for ulcers, gastritis, colitis, and other inflammatory bowel problems. It is high in fiber, and so helps normalize intestinal action; it can be used to relieve both diarrhea and constipation. It may also help alleviate nausea and vomiting in pets suffering from non-GI illnesses, such as kidney disease. A syrup made from Slippery Elm Bark can be used to help heal mouth ulcers from all causes (see recipe below).
To give internally, mix about 1/4 teaspoon of Slippery Elm bark powder with cold water for every 10 pounds of body weight. For very small dogs, it is fine to use the same 1/4 teaspoon dose. The bulk powder may be very fluffy, so pack it down as much as possible to measure it. Alternatively, use 1/2 capsule (per 10 pounds), opened and the contents mixed with water. Slippery Elm powder will absorb many times its own weight in water, so be sure to add enough to make a moderately thick gruel. This gruel can be given before meals by syringe or eyedropper, or added to baby food, canned food, or a homemade diet. It has a slightly sweet taste and is usually well-tolerated by cats and dogs when mixed with food. Give a dose 5 to 30 minutes before meals for sore throat, and before or with meals for digestive tract problems, such as inflammatory bowel disease, until symptoms resolve. (NOTE: Slippery Elm may interfere with absorption of medications; and long-term use may have some effect on nutrient absorption. It may be best to give Slippery Elm at a different time, separate from medications. Please discuss use of all supplements and herbs with your veterinarian.)

BOVINE COLOSTRUM
Colostrum has a very specific effect on the gastrointestinal tract and I have found that it improves the integrity of the intestinal mucosa better than any product I have ever used.
It facilitates the balancing of the intestinal flora, stabilizes the immune status of the gut, and aids in digestion and proper motility of the bowel. Since 70% of the gamma globulins come from Pyers Patches in the intestinal tract, and 90% of all toxins, bacteria, viruses and fungi enter the body through the gut, it only makes sense that we should concentrate our efforts on this organ system.
I have used it for IBD, gastroenteritis of all kinds, hemorrhagic enteritis, colitis, non-specific diarrheas, absorption deficiencies, pancreatitis, constipation, obstipation, and food allergies. It has helped in all of these conditions and has no adverse side effects. It is not a panacea for these problems, but it is very effective in aiding the healing process necessary for resolving these conditions. I have noted that homeopathic cases of mine, that I felt were properly repertorized but had not responded, were now responding with the addition of colostrum. http://www.dogsnaturallymagazine.com/bovine-colostrum/

ALOE VERA
Internally Aloe Vera juice can be added to your dog’s food to help soothe digestive irritation i.e. irritable bowel symptoms especially if associated with inflammation. It functions as a digestive tonic by soothing the lining of the stomach and intestines, and maintaining healthy intestinal bacteria.
If you choose to use the fresh leaf only use the inner gel and avoid the bitter inner rind and its yellow juice as it contains a powerful laxative. The best source of Aloe Vera is a freshly cut leaf from an organically grown plant. Otherwise, always choose a reputable brand that doesn’t use herbicides or pesticides in growth of their crops.
http://www.dogherbalremedies.com/aloe-vera-for-dogs.html

BONE BROTH
Bone Broth Promotes A Healthy Gut
The lining of the intestines is contains millions of tiny holes that allow the passage of digested nutrients to enter the body. Stress, poor diet and bacterial overgrowth can cause more holes to open or to become bigger…this is called leaky gut.
The problem with those big holes is that things can pass through that aren’t meant to, including undigested food matter, toxins and yeast. The body will notice those undigested food particles as foreign invaders and start to attack them. This is how allergies and food sensitivities develop.
Bone broth is loaded with a gooey substance that can plug up those leaky holes: gelatin!
Bone Broth Is Great Nutrition For Sick Dogs
Have you ever had a dog with terrible diarrhea and had trouble getting him back on solid food? Or a dog who is convalescing and doesn’t have a great appetite but you know he needs more nutrition?
Bone broth to the rescue!
Studies conducted in the 1800′s showed that when there is plenty of gelatin in the diet, the body’s need for protein from meat sources can be reduced by as much as fifty percent! Bone broth is also an excellent source of important minerals and can bolster the immune system (think chicken soup)!
Bone broth is also loaded with glycine, which aids digestion by helping to regulate the synthesis of bile salts and secretion of gastric acid.
https://www.dogsnaturallymagazine.com/reasons-your-dog-love-bone-broth/

Arsenicum album (White arsenic)
One of the premier remedies for the gastrointestinal tract. Characterized by violent nausea and diarrhea, foul/putrid diarrhea, diarrhea containing undigested material, pain in the abdomen. Conditions accompanied with restlessness, extreme chilliness, and a thirst for sips of cold water that are vomited. Suddenness of symptoms, prostration, weakness, and collapse. Severe burns.

GOLDENSEAL
http://theanimalherbalist.com/?page_id=174

GrapeSeedExtract
http://ottawavalleydogwhisperer.blogspot.com/2013/11/grape-seed-extract-alternative-medicine.html

L-GLUTAMINE
Gastric Problems Further investigation showed that l-glutamine is also helpful for gastric disorders, such as Irritable Bowel Disease (IBD), colitis and Irritable Bowel Syndrome (IBS). L-glutamine helps to stop inflammation of the small and large intestine and promote new cell growth in the intestinal lining. Using l-glutamine for this purpose does not have an immediate affect, but it promotes improvement over a period of weeks. In dogs prone to these conditions I would continue to use it for a few months even after symptoms have disappeared to continue to promote healing. It has even been found useful in helping ulcers to heal. For more information on dealing with these gastric problems:
http://b-naturals. com/Jul2002. php