Hi MaggieLynn, if it is truly a missing scale, you have nothing to worry about. He probably bumped into something and lost the scale. Nothing needs to be done to treat this as his scale will grow back. Just be sure that his tank water is clean but keep an eye on it to make sure it doesn't turn into a sore. Look around his tank to see if there is anything protruding and "sharpish" that may have cause this. If you are really concerned still, photos are always helpful to rule out other potential problems.