I think that was someone former pet and they did a very poor job at it, none the less that they abandoned the poor litle guy to a very likely doom if you hadnt shined in. Good job in rescuing him
I sugest you give him plenty of great foods, they have a diverse diet but can be extremelly picky as well. Dark leafy greens like turnip, collard, mustard and chicory greens, water cress and dandilions are great staples and really what you want him to eat. Fruits like figs, strawberries, blueberries, peaches and grapes are great treats as well. Slugs, snails, crickets, superworms and high quality turtle and tortoise foods (like from mazuri) should be regulary offered as well. Use a good reptile suplement to help him get that shell rock hard, reptivite is good. Use a 5.0 uv ligth and if he gets better, make a nice outdoor pen for him.