I have had bouts with dehydration too...according to my Dr, a diabetic gets dehydrated when their sugars are running high...you don't say what his blood sugars were when this happened, but, I would bet that they were high...I'm surprised his Dr didn't explain that to him...did the Dr give him a reason for his dehydration??
In humans, dehydration can be caused by a wide range of diseases. Also, dehydration is not a sudden process, it occurs gradually (if no bleeding,diarrhea, or vomiting exists) although when it reaches a certain point symptoms develop. Speak with your husbands doctors and ask for tests to help determine the root cause of his dehydration.