What size is your tank?
What temperature is your tank?
Does your tank have a filter?
Does your tank have an air stone or other type of aeration?
Is your tank heated?
What tank mates does your betta fish live with?
What type of food do you feed your betta fish?
How often do you feed your betta fish?
How often do you perform a water change?
What percentage of the water do you change when you perform a water change?
What type of additives do you add to the water when you perform a water change, what brand water conditioner?
Have you tested your water? If so, what are the following parameters?
Symptoms and Treatment
How has your betta fish's appearance changed?
How has your betta fish's behavior changed?
When did you start noticing the symptoms?
Have you started treating your fish? If so, how?
Does your fish have any history of being ill?
How old is your fish (approximately)?
Copy and Paste that into your reply and answer as many questions as you possibly can.
You should never add any salt into your tanks unless you are treating any type of illness and those two salts are Aquarium Salt and Epsom Salt. Both of those have very strict guidelines on how to be used so don't just dump any of it into your aquarium or they can cause damage.
This "sea salt" do you mean the kind we cook with or do you mean ocean/reef salt? Both are horrible for Bettas. Bettas are freshwater fish, so they do not need ocean/reef salt, and cooking salts are meant to be used to cook with, not for fish, this can cause them serious problems.
The minimum tank size for a Betta should be 1 gallon, any smaller and they can become very ill, very fast. With a 1 gallon aquarium you should be doing every other day water changes, anything smaller should be cleaned daily, which can stress the fish out.
Bettas can go just fine for a week+ without eating and can live for a month without any food. They should not be fed Bloodworms as their main diet, Bloodworms hold no nutritional value at all and are used as a snack, just like chocolate is for us. You should get him flakes or pellets. Since you have him in such a small tank I recommend feeding him pellets as flakes will make the water cloudy. Aqueon, New Life Spectrum and Omega One are good Betta food brands. I prefer Aqueon the most since their pellets are not huge like the Omega One brand.
If he is not interested in feeding don't try and feed him for 2-3 days. After you should try giving him a single pellet and see if he will eat. You can also buy frozen foods for him, these are much more nutritious and tempting for a Betta. Look for Glass Worms or Brine Shrimp. Brine Shrimp shouldn't be fed more then three times a week however as it's a natural laxative and can cause some internal issues. Glass Worms can be fed daily though and are very good for Bettas as that's what they would eat in the wild (since it's mosquito larva). It will come in a little cube in a pack of roughly 20+ individual cubes. Pop one cube out into a small jar or cup, let thaw (may take 15 minutes). Use a CLEAN pair of tweezers and grab a small amount (about the size of a pellet) and place on the surface of the water. If you haven't tried feeding him for a few days he should eat this with no problem.
If he's stressed out go to your local pet store and buy some API Stress Coat+, this is a water conditioner and a stress reliever, one cap full will be plenty. Use this in place of your usual water conditioner.
Try not to bother him for a day or so. Sometimes one of my boys doesn't like me being around him, but once he gets hungry after a day or two he'll come up to greet me.