May sound like a stupid question but are you putting the food in the water with him? Turtles have to be under water to eat because the don't produce saliva... Otherwise, I'm pretty much in the same boat except with a snapping turtle who's actually showing signs of something pretty seriously wrong.
-----
0.1.0 Colombian Red-tailed Boa (100% Het. for albino)
0.0.3 Western Hognose Snakes
0.1.1 Plains Garter Snakes
1.0.0 Red-sided Garter Snake
1.2.0 Barred Tiger Salamanders
0.0.1 Red-eared Slider
0.0.1 Common Snapping Turtle