What's wrong with my turtle?

Sorry to tell you, this is a serious condition. With all sincerity, you need to get him to a rep vet ASAP. No amount of advice you get on Q & A, no matter how experienced the answerer is, and or what advice he/she offers can ensure the health of your turtle. This is a symptom of malnutrition and/or lighting problems and is past the prevention stage and now in the chronic stage. If you can't find a rep vet in your area, send a message via my profile of where you live, and I will see what I can find. I have kept hundreds of turtles as pets. Please trust me on this one for the sake of your turtle. Only highly experienced turtle keepers can help you on this one (and it takes even more than a two page answer). With all my years experience, even I would take him/her to a rep vet. I know this is not what you want to hear, but it's the truth. If you take answers for advice, you are going to take chances of your turtle's survival. I wish you the best. Sincerely.