Definitely agree with above statementIf its possible to keep him outside during the summer, that may help, too. I've heard that turtles tend to do much better in outdoor ponds.
You could just get a 300 gallon rubbermaid stock tank, and set it out beside your house. This would be realitively easy to cover against preadators, and heat when it is cool out. Maybe your turtle is deficient on calcium, or some vitamin. You could try fortifying its food as well.I wish maybe one day I can have an outdoor pond