Hi, you've been given some good info already, I have a few questions and I'd like to add some suggestions.
How old is your beardie, I know that you've had him for 4 months, if you don't know his age, what is his length from nose to very tip of tail?
What size tank are you using?
It is very important that you get a thermometer so that you know what the temps are because that will affect his total health especially appetite and pooping. Since he isn't eating and only pooping once a month, that is not good and the temps are probably a good part of the problem. The only 2 ways to get a good reading are with either a temperature gun or a digital thermometer with a long wire and probe on the end. Here's a picture of the AcuRite brand that I got at Walmart for about $12.00.
Since this one has a double readout, you can get the basking and cool side temp at the same time and it also has a humidity readout which is good to know. To get the readings with a dual readout digital with probe, you place the probe end so that it is resting (laying) right on the basking log or rock and place the thermometer part on the bottom of the opposite side of the tank. You wait about 45 min for the temp to stop rising, then the "out" reading will be for the basking side and the "in" reading for the cool side. If you get a single readout digital with probe, then you would have to wait about 45 min to get each reading separately by moving the probe end from the basking to the cool side so the dual one is much quicker and easier.
Since you haven't been using a
UVB light, that can also be affecting his appetite, etc and a UVB is very important to help their bones develop to prevent metabolic bone disease. Please don't a coil or compact bulb as those are very dangerous. Reptileuv is a good place to get the Repti-Sun 10.0 fluorescent and here is another very good place to get one at a reasonable price.
http://www.petmountain.com/product/fluorescent/504983/zoo-med-reptisun-10.0-uvb-bulb.html Petmountain is also giving a 5% discount off your order right now. Make sure to stay away from the Repti-Glo 10.0 fluorescent as there have been some health issues involved with that bulb. For the hood for the Repti-Sun, you can go to Walmart, Lowe's or Home Depot and buy a standard fluorescent light, remove the plastic cover and the bulb that came with it and replace with the UVB, leaving the plastic cover off. With that bulb, it should be 6 to 8" from your beardie and the UVB should be right alongside a bright white basking bulb so that your beardie can get the benefit of both bulbs while he is basking.
herpfreak gave you the links to the calcium and vitamin supplements that most of us use here, the Rep-Cal brand. If your beardie is under 1 1/2 yrs old, you should be dusting his live food 5 days a wk, 1 feeding a day with the calcium and with vitamins the other 2 days a wk, 1 feeding a day.
Keeping your beardie well
hydrated is very important because if the temps are off, it can cause rapid dehydration. Giving baths 3 to 4 times a wk in warm water tested by the inside of your wrist with it not being hot or cold with the vent area (where they poop) right down in the water for about 15 to 20 min each time, will properly
hydrate them. Be sure to put a washcloth down in the sink or transparent container to keep them from sliding around.
Mealworms aren't a good feeding since they don't really have any nutritional value and due to their hard chitin shell, they are a risk impaction. Crickets, hornworms, phoenix worms, silkworms or butter worms (although the butters are high in fat though) are all better feeders. The link to the beautiful dragons site that herpfreak posted is very helpful for the best greens/veggies to feed and most of us go by that too.
I'll bet you'll notice a difference once you make these changes. Please fill us in on the unanswered questions to fill us in more.