Are you saying you have this turtle mixed in with a variety of amphibians? The toxins exuded by the amphibians can make the turtle sick. If it were me I wouldn't mix a reptile in with amphibians, not worth the risk & that could have something to do with his lack of good appetite.. another thought:
Have you ever had this little guy tested for parasites? It's always a good idea to do that at least once a year.

PHRatz