Has he been checked out for pouchitis (ie inflammation of the internal pouch)?
The main symptom for most patients is an abnormally large number of stools: an increase from 4 to 6 (on average), or more. Patients may also experience rectal bleeding, abdominal cramping, a sense of "urgency" before having a bowel movement, and fever. It's usually treated with antibiotics such as cipro and flagyl.
The large intestine is responsible for absorbing water and salts, potassium and magnesium.
Signs of dehydration include:
Thirst, dark urine, decreased urine, Sleepiness or tiredness, muscle weakness, headaches, dizziness and lightheadness.
A lack of salts and potassium can lead to painful cramping (esp in the calves of the legs while sleeping).
If the cause isn't found, maybe it's worth thinking about switching to a permanent ileostomy instead?
Good luck.