When Lucie went through a phase of being really constipated I gave her a little bit of prune, or prune juice in her rice cereal. I wouldn't give too much though. I actually found the canned prunes really soft and easy for her to eat - just be sure to remove the pip. They are a lot softer than the bags of prunes. Then I actually de-pipped the rest and froze them in an ice cube tray for later use (JIC!) so I didn't waste an entire can. Or maybe just try and give him a bit more fruit to get things moving? Like stewed apple?