Can you post a picture of him? Many of the bettas we see here that have been described as having "fin rot" actually do not have it at all. Due to overbreeding, stress, and their confinement in small spaces, many bettas develop destructive neurotic behaviors. This is similar to dogs and cats that have been in a kennel too long without stimulation--many of them overgroom themselves and walk in circles compulsively. Bettas do the same thing, in the form of glass surfing, neurotically swimming back and forth across the same area over and over, and tail biting--self mutilation of the fins. If the fins are ragged, tattered, and uneven, it's likely that the fish has been biting himself. Many owners never see their fish do this because their presence is usually enough of a distraction to stop them from doing it.
Fin rot, on the other hand, usually originates from ammonia burns on the delicate, thin tissue on the edges of the fins. The burns caused by the ammonia develop a secondary infection--fin rot--which turns the edge of the fin black, gray, and crusty or fuzzy. The edges are usually smooth and the infection usually moves quite slowly.
Wounds inflicted by the fish biting himself can be infected with fin rot, too, although if you're keeping your water clean and doing proper maintenance, it shouldn't happen. Look for the dark coloration on the edge of the fin to determine if he is actually infected. Clear or whitish edges are a good thing--new fin tissue often comes in clear or whitish and colors up later.