Hello,
I am sorry about your dragon. He does appear to have severe metabolic bone disease. While he can recover his health & begin to move around, etc, I don't think his bones will straighten much. He may have the hump on his back permanently as his bones have grown incorrectly.
As suggested, it is possible it could be an impaction since you mentioned that he does not go to the bathroom often. However, if he is hypocalcemic, then, that affects the ability of them going to the bathroom, also.
The Zilla T-50 bulb is a
UVB light, yes. However, it is just plain garbage. The wavelength of the UVB produced is not proper & does not allow for the D3 synthesis to occur, meaning, he is not able to absorb his calcium or vitamins.
The best UVB you can purchase for him in the US is the Reptisun 10 tube bulb and you can get it from petmountain.com
Do not get a compact or coil light though. The tube bulb needs to be 6-8 (no further) inches from him, placed alongside a bright white basking light for him. It shouldn't have any plastic or screen in between him & the light & needs to be mounted directly overhead.
Perhaps you can get some liquid calcium for him, instead. Are you using powdered calcium? What brand do you use?
You can start doing some water therapy on him to help strengthen his muscles & joints again. If you have any time, try to take him out in the direct sun for a short stint, not letting him get overheated. He can still get UVB in the shade, also or on partly cloudy days.
Let us know how he is doing.
Tracie