A. Sounds like a severe gastroenteritis. bloody diarrhea and not being able to retain water means your dog will dehydrate to dangerously lethal levels within less than 12 hours, not mentioning the loss of blood.
You need to take him to the vet as soon as possible as this condition is severe and needs to be treated immediately.
It could be a severe infection, intoxication or a foreign body ingestion injuring the intestines.
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