C.M.
When we started cereal we had the same problem with rice cereal. It caused DD constipation every time. We did two things. One we did not give anymore rice cereal. Second we increased P fruits as we introduced more cereal. The doctors suggested that not only prunes but also pears and peaches would help keep her regular.
One other thing though....look at the incredients of the rice cereal you are using. Many companies add other stuff to the cereal. It is possible that some additional ingredient is the real cause of the pain he is experiencing. We found Beechnut was always the most straight forward ingredients. It's harder to find but worth a try.