As long as he's not showing any other signs of ill health such as weight loss or dark color (especially a
black beard, which can mean he's in pain if it's not accompanied by head bobbing or other behaviors that could mean he's hormonal), then it could be brumation, given the time of year. Every beardie is different when it comes to brumation -- some go to sleep for a long time (up to several months) and some hardly do it at all, so I figure it's best to give them the choice and continue to provide a warm basking area as well as some hides, and have food available in case they want it. If he seems determined to go to sleep for a long time, one thing you want to make sure of is that his digestive tract is empty -- in other words, he should have had a good poop since his last meal. Then most people would recommend checking on him at least once a week to make sure he's not losing weight and that he's at least
hydrated (see if you can get him to drink a little water at least once in a while).
If you don't want him to brumate (most people don't believe it's necessary for their health, just an instinctive way of getting through a harsh winter), you can try leaving his lights on for a longer period, up to 16 hours a day. That might trick his biological clock into not thinking that winter is coming.
It's always a little scary because there are other things besides brumation that can make a beardie lose his appetite and become lethargic (I'm assuming that you've got him set up with proper temperatures and lighting, including UV), especially at his age, so it's good that you're keeping an eye on him and trying to make him as comfortable as possible.
Hope this helps at least a little.