J.N.
I would add about a tsp of Karo syrup (I recommend white, but I've seen others recommend dark) to his bottles. When I began adding cereal to my son's bottle he became constipated also. The Karo syrup works very well. You may have to adjust the amount you use depending upon how much cereal you're giving him, but be aware that too much can cause extreme diarrhea, so look out!! Good luck.