C.,
Infants don't need water if they are nursing, as somebody said, and...I think 5 months is still pretty young to start cereal. They don't need it and the constipation proves that his digestive system is not ready for solids. Some would argue that they don't need solids at all in their first year, and that giving them too soon only stresses the system.
Personally, I would stop with the cereal...if you must do solids, move to avocados and bananas...easy to puree and perfect first foods, then move on to other fruits and vegetables long before you worry about cereals. I did this with my last child (I have 5) and she is my best eater, so I'd say it worked well.
Oh yeah, and I do confess that I always give my babies sips of water with their meals, but not enough to say so, just enough to wet the whistle.
D.