A young beardie is very particular about their needs otherwise they won't do well. The signs you're beardie is showing you mean that he is not feeling well. Most of us here have had beardies for many years, raising them from little babies smaller than yours. We have learned to read their "signs" and have made changes to help them get better and to grow up to be nice & healthy. We really want to help you with your little one.
I put the measurements that you posted in the tank conversion chart and it came up as a 18.7 gallon tank. Since you're using a 150 watt basking bulb, I can just about guarantee you that he is over heating. One of my tanks is a 40 gallon breeder with measurements of 36L x 18W x 18H and I only need a 60 watt household basking bulb to get a basking temp of between 102 and 103. On the cool side, I use a standard 25 watt household light bulb to get a temp there of about 80 degrees. If a beardie is over heating, it can cause rapid dehydration and even heat stroke. The stick-on thermometers only measure the temp of the glass when what you really need to know is the basking SURFACE temp and the only 2 ways to get a good reading are with either an infrared temperature gun or a digital thermometer with a long wire & probe end like the AcuRite mentioned above. As Brandon mentioned, if you're getting a reading on the hot side of 100 degrees, the basking spot has to be MUCH hotter and that is a real concern. If you don't have a Walmart near you, please let us know and we can discuss where else you can get one that would give you accurate readings.
With your aged beardie, it's best to dust his feeders 5 days a wk but only 1 feeding a day with calcium and dust with vitamins the other 2 days a wk but only 1 feeding a day. Can you fill us in on the brands of calcium and vitamins you're using?
How is he doing today? How do his eyes look? Has he eaten for you?