Honestly, in my opinion, this isn't brumation (it could be, but based on everything I'm seeing and that has already been pointed out I don't think so; if you make these important changes that have been suggested and his behavior doesn't quickly change then it may be brumation or something else but I really don't think so), but rather a lack of adequate
UVB light and temperatures that are too low. Do you have a digital probe thermometer or a gun? If not, you need to run to Petco or PetSmart and spend $10 on a digital thermometer with a probe on a wire. The round, gauge thermometers as well as any other "stick-on" thermometers are very cheap and tend to be off by up to 20 degrees! And you can't take his main basking spot temperature with a stick-on thermometer anyway. So I'd go buy a digital probe thermometer and measure the basking spot, then the ambient Hot Side (put the probe on the floor of the Hot Side that surrounds his main basking spot), and the move the probe to the floor of the Cool Side and measure it. Make sure you allow the probe to sit in the spot you're measuring for at least 20-30 minutes each time before you read the temperature to ensure an accurate temperature reading. As already mentioned, his basking spot should be between 98-103, his ambient Hot Side between 88-93, and his Cool Side between 75-80. Temps too cool will make him very sluggish, lethargic, and cause problems digesting his food. Temps too hot can actually cause much the same symptoms...
Now the most important item you'll ever buy for your beardie is his
UVB light. Their entire lives depend on getting both the adequate wavelength UVB and the adequate strength
UVB light. And you have to make sure that you replace his
UVB light at least every 6 months if it's a T8, and at least every year if it's a T5. You also must make sure that his
UVB light is within the correct distance of his main basking spot, otherwise he won't get enough direct
UVB light, regardless of the reflector, and if you buy a T8 UVB tube IT CANNOT SIT ON TOP OF A MESH OR GLASS LID, as the mesh lids block between 35%-50% of the
UVB light (regardless of the mesh hole size), and glass blocks 100%. If you buy the 10.0 T5 High-Output UVB tube then it is strong enough to sit on top of a mesh lid, but never glass... So most T8 tubes must be within 8" of his main basking spot, and most T5 tubes within 11" of his main basking spot. You need to check the long, flourescent tube fixture you buy to make sure it has no clear, plastic safety cover on it that covers the UVB tube, as
UVB light does not at all penetrate glass or clear plastic (no
UVB light comes through the windows of your house at all). So please buy the Reptisun UVB tube and matching flourescent fixture as soon as possible, preferably the Reptisun 10.0 (never buy a 5.0) T5 High-Output UVB tube, either an 18" or 24", get it mounted inside if a T8 and on top of the mesh lid if a T5, get it at the correct distance from his main basking spot, and also make sure that his UVB tube and his basking light are right alongside each other and directly over the main basking spot so he gets both while basking.