What foods are you offering and what was he eating? Also a general run over of caging lighting etc if you could.
If his body has thinned out you may have to resort to tube feeding which is pretty much holding his body fully upright and placing a tube halfway down his throat past his mouth and gradually pushing foods into him - not a fun thing to do. a vet visit is a must as well.
I highly suggest going to anapsid.org, there is lots of info there that will help.