If this was due to dehydration BOTH the hematocrit and the hemoglobin would be high (basically, a lower blood volume from severe dehydration concentrates the blood cells). But, if the hematocrit is normal but the hemoglobin is high, then it is NOT dehydration, and he needs to look at something else, respiratory, cardiovascular, etc.
You cannot assume that everything is j-pouch related just because of the surgery. Sometimes it is, but having a j-pouch does not make you immune to other diseases.
Good luck prodding him into paying attention. Most people do not realize how much lung function or cardiac function can be lost before you have outward symptoms.