Hi, your betta seems in really bad shape. did you post this in the betta emergencies forum? If you haven't please do post there, as there are many who are much more knowledgeable of diseases than I am.
You will probably need a medication for both internal and external parasites a this point, since we do not know which one is affecting him. I am however concerned that there is a possibility of internal infection. have you been feeding him any live foods lately?
I've been recommended API general cure for both internal and external parasites. it is widely available at prety much any pet store. each tab 1 believe is for 10 gallon. you will simply have to dissolve it in a separate measuring cup of conditioned water first. then pour 10% of that into your 1g tank
also, do you know your water temperature?
The salt will not hurt him if used for less than 10 days at 1tsp/gallon