:sad: These guys are hard to keep... It sounds like its suffering from an internal infection of some sort... probably bacterial. Our sand is a lot dirtier than sand in the wild and as such there is a lot more chance for bacterial infection. When these guys dive in for the night they run the risk of infection. You have to keep your sand pristine for them. Also you have to get very lucky. They will sleep in the same spot every night which will help spot clean since they are stirring it up... but if they pick a bad spot to begin with its game over.
He's probably too far gone but I would recommend removing him to a qt tank and going for wide spread gram negative anti-bacterial meds...