I have never heard of this before.
How long have you had this fish ?
Any other fish kept with it, if yes, do they have the same symptoms ?
Any illnesses in the tank recently ?
Do you do regular maintence such as gravel vacuming and water changes (not just replacing evaporated water, but bigger changes)
Any stringy white feces or anything unusual ?
Have you added any new :
fish
changed anything in the tank like decorations or new tankmates,
new brand of food (even new bag of same food you always feed).
Try this first :
Do a good vacume of the gravel and a 50 percent water change. Then buy some cooked (or raw) shrimp in the grocery store and some freeze dried krill from the petstore. Slice up the for him and see if he will eat it or not.
If it still wont eat then :
If you cant find a definete problem and none of the above apply and the fish had been healthy before all this then i would try to get a internal/external parasite medication and treat for it. Alot of skinny fish have picked up a parasite in their gut and that is why they are skinny and off their food. If your fish responds to this then i would treat the whole tank and other tankmates since the others probably have it too.