I've been doing catch up, sorry it took so long to get here. I took the time to look up the ingredients listed for betta revive...
Ingredients: Water, neomycin sulfate (<10%), methylene blue (<0.5%), proprietary polymer mixture, buffers, EDTA, malachite green chloride (<0.01%), cyanocobalamin and electrolytes.
That is probably the worst medication to be using right now. Malachite green and bettas don't mix well, and I wish all of these companies would stop throwing things together and calling them useful. I have been doing research and hands on study of bettas and medications for over 10 yrs now, and malachite green is one along with copper that I will say "don't" use it!
I agree with a lot of what has been said thus far... lack of heater, (80 degrees farenheit is the best for a betta) lack of proper nutritious foods, (I'll make a list for you down below), lack of water changes... all of those things will have contributed to this condition.
I have seen the swollen head problem a few times, but it's not common. Most bettas die before that happens. That is an indication to me of lack of nutrition over anything else. As the body continues to try to function normally, without the right balance of nutrition, the organs begin to fail, and that leads to the outward symptoms that you will see as "a sick fish".
I wish I could be as bright as flashygrrl about hope of recovery, but I'm too honest to spout it at this point. Personally, if it were my situation to deal with, I would humanely euthenize that fish to avoid any further suffering, and then make sure to have all of the proper supplies and information before trying to keep another betta. I really can't even offer you a safe med that will fix a problem this severe... it would be a waste of your money and a lot more suffering for the fish. The damage at this stage is permanent, he can't fully recover, if there is any hope at all of survival.
Before you try this again, please pay attention to the specific needs of this type of fish.
Temp 80 - 82 and stable, which means a heater is mandatory, no matter what climate you are in. As was pointed out, temps fluctuate with day/night cycles as much as with seasons, and those 1 - 2 degrees we may take for granted can mean life or death to a fish. If it happens often enough, end result is usually death, or permanent damage that leads to an early death.
clean water, that means 100% water changes at least twice/wk if 1 gallon or less of water, once/wk minimum if over 1 gallon. Bettas don't like most filters, the flow of the water is too strong for them. If doing the needed water changes, a filter shouldn't be needed at all. I have kept only 3 bettas in filtered tanks over the years, and those were the ones I had the most health problems with.
variety of nutritious foods... live black worms, brine shrimp - either frozen or live are fine, live mosquito larvae, betta pellet food, and even frozen formula 1 food is good. Most bettas won't take flake food, and it serves more to pollute water than feed the fish. I avoid flakes completely.
(I don't use peas, have never used peas, don't suggest peas for a betta. Giving a laxative to a fish, especially when it isn't a part of their natural diet, is a dangerous game to play. That sort of thing can cause internal damage and digestion problems. Bettas are meat eaters, they need the protien. Their digestive tract is not designed to handle vegetable matter/roughage, and that is why it works so well as a laxative.)
Many of my bettas have eaten small rams horn snails for part of their main diet, and I've had a few females who insisted on feeder guppys. When feeding a betta, think meat... protien... and pellets only if they are specifically for bettas, so as to eliminate the additives other fish need and bettas don't.
Don't forget to provide shelter in the tank for a betta. They need places to hide just like any other fish, and lack of shelter/decorations can cause extreme stress long term.
That's really all there is to keeping a betta... it doesn't have to be difficult or expensive if you do it right from the beginning. Most betta tanks won't ever cycle unless there is excess food or snails in the tank. Bettas don't eat a lot at a time, therefore they don't put out as much waste as most other fish... and please... don't overfeed! If you wish to feed more than once/day, make it only 3 - 4 pellets and then a very small amount of something else during those feedings. They have very small stomachs and need time to digest their food. Most claims of constipated bettas comes from overfeeding. Yes, many bettas will stuff themselves if you let them...
And get rid of the freeze dried bloodworms. As was mentioned, that is like feeding them candy... and its not something they need. There are too many healthy options out there to be using something like that. Even a fussy eater will find something meaty as a good food source, it's instinct. A fish that refuses to eat after a week is usually a fish with some sort of illness problem that needs to be dealt with and is causing loss of appetite.
I hope this helps, if you have more questions, please, ask away... we would rather see you do it right and not watch helpless animals suffer needlessly.