Hi there and welcome to Just answer!! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
Most often, this diarrhea is caused by an infection of Trichomanas that will respond to Metronidazole. This is a prescription medication that you can get from your vet.
For now, the best thing you can try as a home remedy is Pepto Bismol that is commonly used in horses to control diarrhea. There are equine preparations for this but using the OTC product , you would give 1 tablespoons per 100 pounds of his weight every 8 hrs. It is also good to give a high fiber supplement to take in some of this fluid.
Good Luck! Sincerely, XXXXX XXXXX