Chuck Norris has lots of problems

I've got two turtles, Chuck Norris and Swan, and I got both of them in February. Swan is doing really well and has grown heaps, but Chuck Norris hasn't grown at all.

Chuck Norris is also really skinny (which seems odd for a turtle, but he is) and he seems to have difficulty moving too much. His shell is really soft, and now it's started to fall off in chunks.
Any idea what could be wrong with him?