You should take him to the doctor. My son also became constipated around that age. He was my first child, so I didn't always know what was the normal amount he should be going, but his poop was also very hard, and he started acting like he was having trouble sitting up. I was concerned that him no longer sitting was a problem with his back, and took him to the doctor and they did an X-ray. Turns out, his back wasn't the problem. The X-ray showed he was full of poop, literally. He wasn't sitting up because of the discomfort in his tummy. The doctor gave him a prescription stool softener in a tasteless powder form that we could put in any of his food or drinks, and that did the trick. He had later had allergy tests which ruled out food problems. He is just prone to constipation, and we periodically have to give him his stool softener. He also drank a lot of milk, and we cut back on how much we let him have every day. But yes, you should definitely get him checked out by his doctor.