I can't see much in the photo that you posted, do you mean those two U-shaped areas at the end of his caudal (tail) fin?
To differentiate between fin rot and tail biting:
Fin rot - look for dark, discolored or "smudged" edges.
Tail biting - look for U-shaped chunks missing, with no discoloration.
Most cases of fin rot can be cleared up with just lots of clean, warm water and good nutrition. Keep the water temp in the 77-82F range, and feed him quality protein-based foods. Monitor for signs of infection. If it's healing up, you'll start to see areas of clear or white growth.
If it's tailbiting, you can try to determine if something is bothering him. It could be anything: stress, boredom, something in his diet or environment, or the weight of his fins. (For example, I have a betta who will tailbite if he doesn't have complete darkness at night. Every now and then, he decides his fins have grown long enough, and he simply rips them off. Yes, I have a betta who gives himself haircuts.....)