welcome to fishlore!
i have not had any experience with fin rot, but, the salt is probably not the way you want to go. Bettas do not tolerate salt. I would give him a
water change, add some stress coat and prime.
Ccould you post any pics of him?
also i read on your aquaruim info that you do not know about the
nitrogen cycle. This is ver important for your fish´s life so i suggest you read up on that

here is a link :
http://www.fishlore.com/NitrogenCycle.htm
I also suggest you buy an Aquarium tester as soon as you can. The
API master
test kit is used by many on this website and i´ve only heard great reccomendations.
As soon as you buy some the test kit, fill in your aquarium info

good luck-
tony