Rice cereal constipation

My son is 4 1/2 months and our pediatrician recommended we start rice cereal a while ago. He’s in the 13% for his weight (About 14lbs) and so I really want to give him rice cereal for the added calories. However it seems to make him pretty constipated (and therefore kind of cranky). Any suggestions? What have you done for added calories that didn’t mess with baby’s system too much?