Brief Answer:Electrolyte imbalance
with possible dehydration
Electrolyte imbalance with possible dehydration seems cause of his current symptoms.
May be increased urination is leading to both electrolyte loss and fluid loss.
This increased urination frequency doesn't look like related to grade I prostatomegaly
It is also important to see his current sugar levels , as they can cause increased urination.
Look for symptoms like dry wrinkled skin, dry eyes
, increased thirst, irritability and tiredness
Also get renal function tests checked.Urine analysis is also advisable.
Meanwhile keep close watch on fluid intake.Also it would be wise to measure fluid intake and urine output.This will help doctor
Hope this helps.Please ask if there are doubts.Regards.