Is your Reptisun 10.0
UVB light a long, fluorescent tube inside a long tube fixture, or is it just one of the compact or coil bulbs that fit into a regular light bulb socket? If it's not a long tube, then yes, this is the cause, as they are not at all adequate for a Bearded Dragon, they are far too weak, and on top of that if you have the compact or coil
UVB bulb sitting on top of a mesh lid, then the mesh is blocking quite a bit of the already far too-weak
UVB bulb. So this is causing a calcium deficiency, that is causing the MBD.
You need to get him a long fluorescent UVB tube and a fixture with a reflector inside it, and since he's already starting to exhibit the signs and symptoms of MBD, I'd totally skip over the much weaker T8 UVB tubes that have to be mounted inside the tank under the mesh and be within 6" of his basking spot/platform, and that need to be replaced once every 6 months. I'd go straight to the much stronger and more adequate T5 strength UVB tube and a T5 rated fixture with a reflector inside it, as he needs this very strong
UVB light, and this and his bright-white colored Basking Bulb need to be left on for at least 13-14 hours every day. The T5 UVB tubes can sit on top of the mesh lid as long as they are within at least 10-11" of his basking spot/platform, and they only need to be replaced once every 12 months, so you'll save money that way too.
It must be at least a 10% UVB T5 tube, or a 10.0 T5 tube (cannot be a 5.0 or lower UVB tube). The cheapest way to go about this is to order both the 10.0 T5 UVB tube and a T5-rated fixture with a reflector online from a place like Amazon.com or Drs. Fosters & Smith, they seem to be the cheapest places.
You can buy a 22" Reptisun 10.0 T5HO UVB tube on Amazon.com for around $24 on Amazon.com, and they also sell one 24" T5 tube fixture that includes a reflector (crucial and a requirement) for around $28.
Also make sure that he's getting calcium 5 days a week and a multivitamin 3 days a week...