It was me who had that same problem with Grendel. Two vets and a reptile specialist all looked at the x-rays they took of her and agreed that she would never get better. She had an bacterial infection sometime in her past (before I got her since I only had her 2 months and they take a long time for problems to develop) that went untreated. The infection went into her bones and literally ate away at them. There was a blank spot along her spinal column.
With MBD it can sometimes be halted or even made better by feeding liquid calcium to the gecko. I did it for two leos that are happy and healthy one. One actually bred and had beautiful babies.
Grendel was dragging her back end along with her little feet sticking to everything she pulled herself across. She would try to jump only to be pulled back by her feet stickers. I put her down, on the advice of all the vets, since that was no way for an arboreal animal who loves to jump and climb to live. It broke my heart but it was the right thing to do in her case.
Good luck with your little guy.
Randi