I wouldn't say that banannas are bad, but they should only be given every once and a while. As for your red ear, have you tried giving him letucce? I have a red ear, map turtle, & a yellow belly slider in a pond, I throw a leaf of either collard greens, turnip greens,or romaine lettuce in there. The only turtle that eats it is the yellow belly. But I have hope that the others will try the greens some time!
Anyways, all I can say is keep trying, just throw some lettuce in there every once and a while and see if he will eventually eat it. Have you tried giving him mealworms and earthworms? They love those.