Kingofnobbys has answered everything perfectly, so there isn't much I can add except that since he's so small and young (How old is he?) the most important thing that you can do for him is buying him a very good UVB light, either a Mercury Vapor Bulb (MVB), which is an all-in-one bulb that supplies both adequate UVB for a beardie and his heat, or the other option in Australia that is the absolute best option available is an Arcadia 12% or 14% UVB, but it must be the long, flourescent tube version and then a matching flourescent tube fixture in the same length as the tube. Arcadia UVB tubes are the best bearded dragon UVB lights available, I wish they sold them here in the U.S., but we have Reptisun UVB tubes which are very close in quality. A good piece of advice is NEVER buy any bulb that is a "coil" bulb, not for UVB or for basking, because #1 none of them produce adequate UVB light for a bearded dragon, and #2 they all seem to cause serious eye and skin issues. So just avoid all "coil" bulbs. Period.
Once you get whichever UVB light you're choosing, make sure that it is within the correct distance of the main basking spot based on which light you buy. We can help you with that once you tell us which one you buy. Also, make sure that the new UVB light is right alongside the basking light and both are overtop of his main basking spot, at the correct distance from it for the UVB. These practices will ensure that he's getting the correct and adequate UVB light that he needs every day to be able to digest his food, make vitamin D3, and absorb all of his nutrition, including his Calcium, which will stop any developing MBD and get him back on track with his growth and prevent and stunting. ****Be sure to replace his UVB tube/bulb according to what the directions for that bulb are, it's going to need replaced at least once a year, some are every 6 months, because they stop emitting UVB light even though they still turn on and emit light.**""
Since he's young and will do most of his growing during his first year of life, getting the correct UVB light placed at the correct distance (unobstructed by any clear plastic cover that may come on the light fixture and is covering the bulb for safety, remove it if the fixture you buy has one!) and getting all 3 of his Temperature zones within the correct ranges listed above by Kingofnobbys, and feeding him at least 2 feeding sessions per day, 3 sessions is preferred, of live, quality feeder insects, dusted with Calcium and multivitamins, each session lasting 10-15 minutes where he's allowed to eat as many live insects as he wants, as well as offering him fresh greens from the list above. Following all of these instructions will get him back on track and allow him to grow to his full potential without any Calcium deficiencies.
Once you get whichever UVB light you're choosing, make sure that it is within the correct distance of the main basking spot based on which light you buy. We can help you with that once you tell us which one you buy. Also, make sure that the new UVB light is right alongside the basking light and both are overtop of his main basking spot, at the correct distance from it for the UVB. These practices will ensure that he's getting the correct and adequate UVB light that he needs every day to be able to digest his food, make vitamin D3, and absorb all of his nutrition, including his Calcium, which will stop any developing MBD and get him back on track with his growth and prevent and stunting. ****Be sure to replace his UVB tube/bulb according to what the directions for that bulb are, it's going to need replaced at least once a year, some are every 6 months, because they stop emitting UVB light even though they still turn on and emit light.**""
Since he's young and will do most of his growing during his first year of life, getting the correct UVB light placed at the correct distance (unobstructed by any clear plastic cover that may come on the light fixture and is covering the bulb for safety, remove it if the fixture you buy has one!) and getting all 3 of his Temperature zones within the correct ranges listed above by Kingofnobbys, and feeding him at least 2 feeding sessions per day, 3 sessions is preferred, of live, quality feeder insects, dusted with Calcium and multivitamins, each session lasting 10-15 minutes where he's allowed to eat as many live insects as he wants, as well as offering him fresh greens from the list above. Following all of these instructions will get him back on track and allow him to grow to his full potential without any Calcium deficiencies.