Your UVB is not 10%, and it's a compact bulb. I believe in Australia you have a better chance of finding the Arcadia 12% T8 tube than the Reptisun 10.0 T8 tube, but those are pretty much the only UVB bulbs (besides a Mercury Vapor Bulb, which would eliminate the need for a separate basking bulb) that are OK for Beardies. The UVB needs to be a long tube, either 18" or 24" long for your current tank, though if you buy a 6ft long enclosure then I'd get a 36" or even 48" UVB tube to cover the entire tank. I would not use a Mercury Vapor Bulb for a 6 foot long enclosure, as he'll only get the
UVB light in one spot, while a 36" long or 48" long tube will cover the length of the tank if you center it. A T8 UVB tube must be within 6"-8" of your beardie while he's basking.
If you actually are getting a 6 foot long enclosure, I think your best bet for a proper UVB tube is going to be a Reptisun 10.0 T5 High-Output (HO) T5 tube, because they can be up to 12" away from the beardie's basking spot, they are the best
UVB bulb out there for a bearded dragon, in my opinion, and very necessary if you're putting him in a very large enclosure. You can buy the Reptisun 10.0 T5 HO tube
uvb light at
www.petmoutain.com fairly cheaply, for a 6 foot tank (don't know how deep or high this enclosure is going to be) I'd buy the 48" T5 HO. I believe it's actually labeled as being 46" long, but it will fit in a normal 48" T5 fluorescent tube fixture, which you can order on Amazon.com cheaply (it's difficult to find a 48" fluorescent fixture rated for a T5 tube that is 25watts). Reptisun sells a T5 hood but it's quite expensive, and they also sell a package deal that comes with the hood and a Reptisun UVB tube the same size, but stupidly the bulb it comes with is a 5.0, not a 10.0, so that's useless to a beardie. I have the Reptisun 10.0 T5 HO UVB in the 22" length, and I bought both the tube and a 24" T5 fixture rated for 25watts at the same time from Amazon, and I saved a fortune. My girl is in a 4'x3'x2' enclosure, so the T5 High-Output 10.0 Reptisun is absolutely necessary, and most definitely is for you if using a 6 ft long enclosure. The nice thing is that even though you might spend a bit more for the T5 High-Output fixture (although not much if you buy one of the ones from Amazon), the T5 High-Output 10.0 lasts for a year while the 10.0 T8 has to be replaced every 6 months, do the cost evens itself out. And again, the T5 High-Output UVB can be 12" away from your beardie and the T8 must be within 6"-8", so the T5 High-Output is a no brainier for your necessary
UVB light.
If your guy is only 4 months old he's still a bit stunted, but not horribly, and if you can get him under a proper
UVB light (VERY, VERY IMPORTANT FOR HIS APPETITE, DIGESTION, AND NUTRIENT PROCESSING!) and get his basking temperatures correct, his appetite should pick up pretty quickly and his growth may eventually approach close to normal, but I cannot express how important getting rid of the compact
UVB bulb and getting him under a Reptisun tube UVB is. And you have to think in terms of the size of his enclosure in relationship to his lighting. I couldn't tell if you already have him in the 6 foot long enclosure, or if he's currently still in the 2 foot enclosure, but obviously the lights you buy for the 2 foot enclosure will not be adequate for the new 6 foot enclosure, neither the UVB or the basking bulbs. So you're going to need to redo his lighting to get an appropriate temperature gradient in the 6 foot enclosure. He should have a cool side between 75-80 degrees Fahrenheit, a hot side between 95-98 degrees Fahrenheit, and a main basking spot between 105-110 degrees Fahrenheit (lower depending on his age, subadults and adults like their main basking spot a bit lower between 100-103ish). Then depending on the height of this new huge enclosure you'll have to adjust his basking platforms, which is why the Reptisun 10.0 T5 High-Output UVB tube is absolutely the best choice, because it puts out enough proper UVB to allow for the basking spot to be only within 12" instead of 6".
Also, please, NO LIGHTS AT ALL ON AT NIGHTTIME, IT DISRUPTS THEIR SLEEP!!! His nighttime temperature needs to drop, just like it normally does in the desert at night, but it needs to stay at 65 degrees Fahrenheit at the lowest. You said his nighttime temperature is around 50 degrees, if you meant 50 degrees Fahrenheit then that's way too low. I couldn't tell what temperature scale you were using at the time. A 6 foot enclosure will obviously drop lower than the 2 foot enclosure, so if the temperature inside your house drops below 65 degrees Fahrenheit at night, need to buy a Ceramic Heat Emitter (CHE). They screw into a normal lightbulb fixture and are rated in watts just like a lightbulb, but they only emit heat, no light. This is a necessity of his temp drops below 65 degrees Fahrenheit at night, however I'd get a lower wattage CHE, like maybe 60-65 watts, because you do want his temperature to drop at night. Again, please, no lights at all at night, especially those ridiculous red, blue, or "black moonlight" night bulbs, Bearded Dragons see in color so colored bulbs will disrupt their sleep.
Once you get him an appropriate UVB tube (most important purchase you make for your beardie, as literally ALL of his body systems and functions rely upon proper UVB; unfortunately there are very few UVB lights out there that are even close to appropriate for Bearded Dragons) and.get his daytime cool side, hot side, and basking spot temperatures within the correct ranges, his appetite will go crazy and so will his energy levels. He should be allowed to eat as many live insects as he wants to in a 10 minute period, and you need to do 2-3 of these unlimited feeding periods every single day. This is very, very important while he's a baby, juvenile, and sub-adult, because if he is not permitted to eat all the protein he wants now, during his growing periods, he will never reach anywhere close to his full growth potentials. So there is really no set number of crickets he should be eating, he needs to get ALL HE WANTS IN 10 MINUTES! For example, I just got a new leatherback baby boy (I think), his name is Dee Dee, and he was just 4 weeks old when I got him 2 Saturdays ago. So he is now just a month and a week old, is about 6.5 inches long, and is eating size small crickets (pinheads were pointless) with 2 small wax worms at the end of his first feeding period each day. He gets 3 10-minute free feeding sessions every day of small crickets (he has fresh greens and small Mazuri Bearded Dragon pellets in his enclosure at all times,he picks at both and I want him to have access to them at all times, but they typically don't eat many greens at all until they approach a year old, and pellets are hit and miss, though he likes them, but only if they're dry, which is odd but whatever). So the amount of greens and pellets he's eating is negligible.
I turn his UVB and basking lighting on around 7:00 each morning and I make sure he's been under them for at least an hour before I give him his first feeding (this is important). So after he's gotten at least an hour or more of
UVB light and he's gotten his temperature up for at least an hour, I do his first 10 minute feeding period with small crickets. He typically eats the most crickets in the morning feeding, usually he eats between 10-15 small crickets during this feeding, as well as his treat of 2 small wax worms. I then put his fresh greens and pellets in his tank. He gets his second 10 minute feeding period between 2-3 in the afternoon, and he again usually eats between 10-15 small crickets. Then he gets his third 10-minute feeding period between 6-7 at night, and he typically eats between 5-10 small crickets. I then leave his lights on for another 2 hours after he has his last feeding of the day. I then take the greens and pellets out. So he is averaging between 38-45 small crickets per day, 2 small wax worms, along with a few greens and pellets. I'd love to feed him medium sized crickets because he'd eat less and it would cost less, but he's still much too small. I'm going to start giving him 1-2 small wax worms after each live feeding, as treats. SOMETHING A LOT OF PEOPLE DON'T DO THAT I FEEL IS VERY IMPORTANT IS ALLOWING THE BEARDIE TO BE UNDER THEIR UVB AND BASKING LIGHTS FOR AT LEAST AN HOUR BEFORE THEIR FIRST FEEDING OF THE MORNING, AND LEAVING THEIR LIGHTS ON FOR ST LEAST AN HOUR AFTER THEIR LAST FEEDING OF THE DAY! THIS IS VERY IMPORTANT FOR DIGESTION AND NUTRIENT ABSORPTION.