Where is he loosing weight?
If it is getting a "knife back" while having a fat belly I would say internal parasites or worse some infection or damage of internal organs.
It could also be a late effect of cyanide poisoning.
Back in the early days of saltwater aquariums (70s & 80s) many fish, especially from the Philippines, where caught with cyanides. Most would survive and recover, but some that got a higher concentration at the center of dispersal would not gain any weight and slowly waste away while eating and initially looking healthy. This is due to serious damage to internal organs, most notably the liver.
Unfortunately cyanide is still used to catch fish:
Cyanide: an easy but deadly way to catch fish
As for mandarins: I had quite a few in the past and hardly ever any problems with them. Most were eating like pigs and even outcompete other fish in the hunt for live and frozen food.
Right now I have a pair of mandarins in a 25 gallon tank and they are doing fine. When I got them two months ago especially the female was very skinny but both gained weight fairly quickly and now they are even spawning. I feed a mix of live and frozen food and they are much better at getting to it than my gobies or the stupid percula (which prefer pellets over anything else).
One of the key things to keep picking fish like mandarins well fed is to have the right rock and gravel in the tank. Manmade live rock, while looking ok and having lots of big holes for housing small fish are not good for giving enough shelter to pods due to their lack of porosity. Real live rock is much better (the best and most porous is Mediterranean live rock - though impossible to get in the US). Another good way to keep a reservoir of pods and other small crustacean is coarse coral gravel at the bottom of the tank. I have an about one inch thick layer that is crawling with life: copepods, amphipods, isopods, even Mysis. You won't get that with fine sand...
