First, do you use plastic plants? If so, take them out for his treatment. They may (or may not - this is just in case) be the cause. Or he could have bitten his fins. Either way, you're going to treat it.
Before treating, look at his fins. Do they look singed? Like someone set them on fire and then put them out? They should be tipped with black or gray. There may even be a black or gray line all the way down his tail. If he does not have this, then it is not fin rot.
Bettafix is a deluted version of Melafix. While bettafix can cause problems, some people have had success. While others have not. Yet another group of people say it does nothing.
Melafix, however, unanimously seems to cause problems everywhere for bettas (which is why bettafix was invented).