This is usually a sign of Metabolic Bone Disease (MBD). Make sure you have a proper UV light on him and are giving him plenty of calcium sprinkled on his food. My daughter volunteers at a local reptile rescue center and recently brought home a pitiful, little beardie with MBD. Luckily, we have a good vet that was able to help us out with the proper lights and powder along with a few calcium injections.