what are the symptoms and possible causes you suspect for his being sick? you might consider whether this illness may be related to, or worsened by possible malnutrition from going so long on a heavy protien diet.
probably the best thing you could do right now, in relation to diet, is try anything to introduce the extra vitamins & minerals from veggies & friut in order to boost immune response. any stress associated with a slight change in diet is negligible at this point in comparison to the illness itself and lack of nutrients. at the least, get some vitamin dust on his meals. continue to offer the food he likes, just try to work in the other things.
also, very important...if you are seeing obvious signs of illness in a turtle, consider it a serious matter. turtles, like other reptiles I've had experience with, mask any symptoms of illness for a long time so as not to show weakness. this is important in the wild to deter attention of opportunistic predators. they will carry their symptoms secretly until they become too weak to keep it up. by the time this is noticed overtly, there is a likely possibility it is more serious than one might suspect. an experienced reptile vet consultation would be recommended given in your words he is "pretty sick".
also note that reptiles can go quite a while without food, as you probably know if you've ever hibernated him. if you normally feed everyday or every other day, and he's not responding to your new diet methods within a week, something needs to be offered or methods modified. do not continue to withhold food to make him hungry. what happens is the reptile will reduce his metabolism and begin to "shut down" what is not necessary for life support. exactly what they do for hibernation.
lastly, UVA & >>UVB