Since the pictures are rather small and unclear, there are a few possibilities.
It's possible he swam under something and simply scraped his scales. If the area looks clean and there's no huge change in texture, I would simply increase the water changes and make sure the scrape does not get infected by keeping the water pristinely clean. You should obviously search for whatever sharp object he swam under that caused the injury.
If the fish is acting lethargic and the wound becomes fuzzy or crusty, it is likely infected and will need to be treated with a broad spectrum antibiotic.
If this is not a scrape or wound of any kind, simply a change in coloration without a disruption of the scales or anything like that, consider that your fish may simply be a marble betta. To learn more about marble bettas please read this article: http://bettysplendens.com/articles/p...articleid=1114