Is the Reptisun T5 High-Output
UVB light you ordered a long tube or another coil or compact bulb? Because it needs to be a long tube, either 18" or 24" are the common sizes, and you'll also need to order a long flourescent tube fixture rated for a T5 tube, which is cheapest on Amazon.com from what I've seen. If you ordered another compact bulb then I honestly don't know if it is appropriate or not for a beardie, we always recommend the long tube UVB lights, so someone else can chime in and let us know, as I've never seen anyone buy a compact version of that bulb. If you ordered the coil version then I know it's not adequate for him, coil bulbs are known for causing all sorts of serious health issues in bearded dragons, the most common issue being severe damage to their eyes, sometimes even blindness. So I hope you ordered the long tube and a fixture for it. The reason the tubes are best is because the fixtures contain a very long reflector and they cover a much larger area of the tank anyway, but with the long reflector in the fixture your beardie will be getting adequate
UVB light (the T5 High-Output tube needs to be 11" away from his basking spot at a minimum, but can sit on top of a mesh lid, it's strong enough to penetrate the mesh, but not a glass lid) not only on his basking spot but anywhere in his tank.
UVB light is so important for beardies, their entire lives are dependent on adequate wavelength and strength of
UVB light for at least 12-14 hours a day, every day, or they cannot make vitamin D3 or process any of the Calcium you're giving him at all. The EXO Terra coil UVB you've been using for a year (should have been replaced at 6 months old, do if it wasn't he's had no UVB at all for 6 months) is very weak to begin with, and this is definitely the cause of his issues without a doubt.
He has classic symptoms of MBD due to a bad
UVB light. Typically their back legs becoming very weak or suddenly partially paralyzed due to weakened muscles is the first symptom you'll see in MBD. If you get him an x-ray tomorrow this will typically confirm a diagnosis of MBD easily, as his bone density will look very transparent. He definitely needs to be on a liquid Calcium supplement for a while instead of a powder, something like Calcivet. Hopefully once he's under the T5 High-Output Reptisun UVB his appetite will pick up within a matter of days, as will his activity levels, and giving him liquid Calcium every day under appropriate
UVB light should stop the MBD development, there's no way to tell how much if any of the issues he already has will be reversed, but it's possible. He's also very skinny from not eating as you've said, so his muscles are also being weakened from being broken down for energy on top of the MBD. So you're taking the right steps to get him back on track.
If someone else can comment on the Reptisun 10.0 T5 High-Output
UVB light in the compact version please do, as I'm thinking that is what the OP ordered to replace her current coil UVB, because she didn't mention ordering the fixture for a T5 tube. I am not familiar with the compact version of that
UVB bulb, obviously the coil bulb is not adequate, but I have no idea about the compact. If you did in fact order the long tube UVB and a T5 fixture for it, then ignore this comment.
Also, the Reptisun 10.0 T5 High-Output
UVB light, no matter what the version, needs replaced every 10 months to a year, as they stop emitting any
UVB light at that point even though they still emit light.