IMO, the best way to treat fin rot is by keeping him in clean, warm water.
Bettas are sensitive to salt, which is why it can only be added for a maximum of 10 days. Exposure to it for too long can harm their kidneys and internal organs. Therefore, I would only use Aquarium salt if there's a reason to add it. (For example, if a bacterial infection has set in.)
How often do you do water changes? If he is in a 0.5 tank, you will need to do VERY frequent water changes in order to keep ammonia and toxin levels low. Ammonia and toxins are colorless, so you can't see them in the water. This means that the water can *look* clean, but still have high levels of ammonia.