We had the same extreme issue! I stopped breastfeeding my son at 17mo and that made it even worst. It took few weeks to find a bottle he would accept. We had to mix milk with water in order to get him more liquids. On hot summery days was easier but on colder days he would refuse to drink at all. Constipation was an absolute nightmare. Fruits, soups, milk with water was not doing the trick. - we tried everything but refused Miralax - We started him on Raw Probiotic that we would mix with his morning milk. He also slowly started accepting blended juices I make. I would blend mango, kiwi, watermelon, plum, oranges, some pineapple - always avoided berries - added water or coconut water and if he had it daily he would poop - hard but it would happen. I absolutely excluded rice and bananas from his diet in any form. Happily he started outgrowing the issue. He is 2, 5 yo now and just this winter we started slowly introducing bananas and rice, very little and not too often. His liquid intakes are much better than months ago but could still be better, he loves milk and we still mix it with water. With the morning dose of the Raw Probiotic things are happening but we still hrld our breaths until the poop comes!