ummm - maybe try cucumbers and melons - since they are mostly water may take the edge off dehydration and get his tummy hungry - one very short term, since there is little nutritional value and they get hooked quick.
Can you dip the pellets in tuna water (can of tuns packed in water) - again only short term.
If all else fails try a tiny bit of canned tuna fish. Again, you have to be careful, or that be all he eats and you will have to break him of habit. so anything you give him keep offering his pellets and greens.
Just talk nice and gentle, approach slowly, basically just be calm so he doesn't get scared.
I'm kind of afraid to say leave him alone and see if he eats since he's flipped on his back twice. Maybe sit quietly just close enough to keep an eye on him,but far enough that he "feels alone" = gets a little boring,but whatever works.
Count the turtle pellets you put in, maybe he is eating one or two when you aren't looking....
Do you have a picture of his set up and stuff on his neck?
He may be ill, which is why he is not eating.
A picture would really help.
