have you had a UVB light on your boxie?
what about supplements or cuddle bones? It sounds to me like a fairly advanced form of metabolic bone disease. Unlike mammals who show signs of illness fairly quickly, reptiles seemingly remain fine until they are very sick. It takes sometimes months or even years for a reptile (especially turtles) to show signs of illness, and when they do it is often too late. Fortunately, though, it sounds like you caught it in time. He is not yet refusing food. Most likely, though, he is in pain, and you should get him to a qulified vet as soon as humanly possible.
He will probably need calcium injections and oral calcium lactate. hope this helps.