7-month-old is constipated. 3 days no poop. Coincides w starting to eat rice cereal, tho doesn't eat much. Have given prune purée. What else to do?

Stop the cereal. Stopping the cereal for now and resume the usual food/milk and your baby should be back to normal in a day or two. If baby is comfortable, do nothing except above. If uncofmortable/straining hard etc. A gentle enema will likely result in a bm in 30-60 minutes. If not better in a couple of days, check with doc. Good luck.
Water. Constipation is usually from too much constipating food, not enough fiber, or not enough water. If he was never constipated before, try a couple of ounces of water once or twice a day. If that doesn't help, consider that what you regard as not to much rice might actually be too much for him. Is he eating fruits and vegetables? Those help a lot.