You might want to try and get a necropsy done. Because it sounds like he had something serious going on, more serious than minor mbd. He may have had a heavy parasite load. He wasn't thin enough that he would've starved to death from lack of eating, and he looked hydrated in your pics. He may have had some serious damage like kidney damage or something from lack of correct care or water source at the pet store. I am sorry for your loss. So sorry.