Try to do more water changes and see if it will help to perk him up. I am not sure how much and how often you change his water before?
Since he is sick i would turn off the filter and try to do more water changes. Do 3- 50% water changes about 2-6 hrs apart. Then do another full water changes about 4 -6 hrs apart from the last 50% water change.
Stop betta fix it very easy to overdose and it harmful for bettas labyringh organs. So do all water changes to get rid of the betta fix and salt.
Salt has antibacterial affect but you need to use right dosage though and duration.
How much and how long you been adding the salt?
Is he still eating?
What food you feeding him with?
If he is eating i would recommend to go and buy frozen blood worms , it enriched in protein and good for the immune system.
What kind of water conditioner are you using?
Did you ever check your water to see if you have ammonia in your tap water?
And you can lower the water so it will be easier for him to swim up for the air and take out the filter for now.
There is a lot of factors that betta can get sick...but i think now you need to try to get him better.
Also i just read Oldfishlady thread after i put my post and it also a lot of helpful information there.