Most probably it is natural brumation or hibernation. It seems unfortunately many beardie owners dislike the fact that their animals get inactive during the winter and try to have them constantly up and interacting. Keepers of other reptile groups, like tortoises, have recognized long ago the benefits of hibernation and intentionally hibernate their animals. The lizard now is reacting to the lesser photoperiod and temperature by hiding and starting sleeping more. If he has a good weight, you can gradually cut all food and heat. Eventually he will fall in a true brumation, where only you need to wey him and trickle a little water on his snout when checking on him, aprox once a week or less. On the coming spring he will be strong and lively again.
ps. Many commonly kept reptiles, like red-eared sliders, leopard geckos, corn snakes are true hibernators as well. They come from temperate climates and not only slow down in winter, but they can safely become inactive. I don't know why the overwhelming majority of keepers and caresheets ignore the fact. I believe if it is natural for an animal and the animal in question is healthy, hibernation should be induced. It might have beneficial effects on hormones and the lifespan.
