M.T.
It could be the cereal, or if you are giving him formula, it can be that as well. At 4 months, he does not need cereal. The AAP doesn't recommend food til 6 months. Rice is very binding, when you give cereal, I'd go with a whole grain baby cereal or baby oatmeal instead of rice. I would try nursing and pumping more - giving cereal and formula will diminish your supply. Also, a bf mom can eat prunes or other laxative types of foods and your baby will get some of the effect
Good luck