I would keep up with the water changes and add a bit of salt. If your fish is darting behind the heater, it may be a sign that there isn't enough cover in the tank for him to feel secure. Often a cave or a plant is not enough, some bettas are more high-strung than others and need more to hide behind. Consider adding more decor to his tank and watch him carefully.
If he becomes lethargic, loses his appetite, or the scrape begins to look infected, you may have to consider an antibiotic treatment in the future.