It's most likely a high parasite load. Wild caught animals are really dragged down by these. I've never had to dose one of mine with meds, so I don't have any good advice for that. I would try searching here for anyone else who had similar issues : http://www.caudata.org/forum/
But keep in mind that if he's weak, the meds could kill him
Some people prefer to wait and see if the load goes down while in captivity.