Hi,
I am sorry to hear you are having so much trouble. I do have some suggestions for you though
(1) maybe don't move te little guy around so much. He's probabaly freaked out as it is. Keep him inside i na a well-planted cage with lots of hiding spots for a while. He'll be easier to monitor that way and he won't have to deal with the stress of constant changes. Maybe after he's eating he can go outside permenantly or learn to deal with the in-out thing.
(2) You might try leaving a partly sunken dish of various food items in the cage (maybe use the cap from a large jar or a small bowl) The turtle may be more inclined to eat if it feels safe. If the people from whom you got it neglected it, perhaps they did worse than that and so maybe he is afraid of humans.m He might eat when you are not around.
(3) I don't know what species youhjve but i know that sometimes Western Box turtles and Ambos won't eat red wigglers ( i think its the acidic smell that also turns off soem frogs and lizards) . Many of these same turtle will however eat large earth worms and slugs. See if that works.
(4) Some other foods that boxies like that you missed: egg white (hard boiled) slugs/snails, strawberries (cut open to release more odor) blackberries, mulberries, beetles of any kind (especially Terrepene ornata)cucumber slices, cat food (not a great choice but good for inducing fussy eater, banana (same note as with cat food)
Also, is there access to a low humidity area as well as a a high humidityy area? This sometimes makes a difference. I have a three toed who will not eat until he has dried out pretty well though he spends all day in the wet part of his enclosure. I wish you the best of luck. I know a declining animal is cause for a very stressful situation. I hope (s)he comes around. BTW what species is it?