personal experience fire eels can sometimes stop feeding on dead foods once over 12" i have know of 3 fire eels to do this in my time... all would only feed on live feeder shrimp from the point the stopped taking dead food!
Have you tried hand feeding him with a pair of tongs. Mine used to slurp down ghost shrimp, so maybe that might get him started or even some small feeders. Just feed the oscars first and then try getting the food to him.