If the legs are twitching it could be a sign of kidney problems. Do you feed anything other than mice? A straight mouse diet is not recommended for these frogs; you need to vary the diet with other things. If you have him in a wet substrate at this point, drying him out a bit might be one way to restore kidney function. Just let the substrate get dry, but make sure you have a water dish available, so if he gets too dry he can hop in it.