OK, desperation mode here. He's been sick a fairly long time and not getting better.
I think he has a bacterial infection called Columnaris, caused by Flexibacter. I think poor water quality is the cause and the toxic byproducts, even at trace amounts, have to be removed by daily water changes.
Water changes to give him pure water will be less stressful than being in water with toxic byproducts ammonia and nitrite. He will feel better just on the basis of having pure water, alone, but more importantly this is needed to help him fight this infection.
I would follow BettafishMommy's water change recommendation. Star_Rider's recommendation about water aggitation to help add O2 to his water. A small filter, without the carbon, will help oxygenate the water as will a bubble stone.
If you can go quickly in the morning to a petstore for Kanamycin (Kanaplex, Kanacyn) this is the best antibiotic for columnaris. Actually medicated food is best but at this time he probably won't eat the antibiotic food.
If you can't find the Kanamycin then get these two: Maracyn and Maracyn II. These two antibiotics together will fight gram negative and gram positive bacteral infections.
Keep the temperature of his tank stable at around 76 degrees or slightly less, cooler than the 78 degrees bettas prefer since bacteria like warmer water.
Each day you must do the big water change, using Prime conditioner to help detoxify the ammonia and nitrite. I seem to remember you have a conditioner that says it detoxifies ammonia and nitrite, so it would be OK to use.
Then dose the tank as directed on packaging.
24 hours later do the big water change and then dose the tank.
Keep doing this every day.
Water changes won't interfere with the antibiotics as they aren't available after a few hours in the tank anyway.
Water changes will be less stressful to him than being in water that is not pristine. Pristine water will help him fight this. Ammonia must be 0. Nitrite must be 0. Nitrates 20ppm or less.
I didn't notice if you have any silk plant in there, but he needs something to rest on close up to the surface of the water, so he doesn't struggle to come to the surface.
Keep the room low light, as much as possible. If he is fond of you spend time with him. Keep the room free from harsh noise.
Water change, dose, 24 hours later - water change, dose. Keep this up for at least 10 days. If it is helping there should be some change that lets us know he is improving. There may not be obvious physical changes, but he may act like he is feeling better. Less lethargy, more swimming.
You will have difficulty measuring the dosages with him in the tank you have. The med is packaged in dosages for a 10 gallon. I needed to use half a packet each time I dosed since my hospital tank is 5 gallons, but the powder is so scanty it's hard to do exactly half. You can't use measuring spoons.
I wound up making lines of powder, like cocaine lines (I've seen movies, lol, I never did drugs), using card stock to pour it out on, and another piece of card stock to drag and smooth and make lines that seemed to be even. Then I cut the line in half and carefully scraped it back into the little foil pouch.
It might be easier to get a 10 gallon sterilite container from Wallyworld, then the packets will be just right.
One way or another this is going to be a lot of work. I'm sorry your betta is sick. You've really been trying to help him, remember that if things take a turn for the worse.
I know it's late now, and the pet stores are closed (wallyworld doesn't have the meds you need), but hopefully first thing in the morning you can get the things you need to get him started on antibiotics.
I hope you little guy pulls through this. We're all pulling for him, and you. Keep working hard, he's depending on you. Keep us posted
