Okay update today:
I just did the daily water change and didn't add anymore salt. For reasons I didn't get to it until pretty late in the day, and my little guy barely moved from his spot all day until I did so. Still won't respond to food. Snail remains blissfully unaware of anything other than what he can climb on and scavenge.
I used a service online to try to get in contact with an actual veterinarian, and after giving a barebones explanation of the situation he suggested antibiotics, and when I asked what kind said Maracyn Two. Though admittedly, based on quite a few points during our conversation I've got some doubt as to whether or not he's actually a fish veterinarian, so I'll take this particular advice with a grain of salt.
I talked with my cousin, and she said that her paradise fish once exhibited very similar symptoms to mine, and she went to the owner of this particular fish specialty pet store near here for advice, and he gave her something to put in the tank everyday that fixed the fish's problem and saved her life. Coincidentally, this is the same fish store I got my betta from last summer.
I'm not 100% sure since she said the problem with her paradise fish had to do with constipation and it doesn't look like that's my little guy's issue, and while that store seems to have expert care of most of the creatures there, their treatment of bettas seems to be the one area they decided to skimp on (quite a few were admittedly in 5 gal tanks with other fish, and most of these were probably females since I saw a lot these tanks had more than one betta in them, but most of the males are kept in tiny dirty bowls right next to each other, like my baby was). Still though, if I'm going to get any proper expert fish advice from anyone it'll probably be the guys in charge of that place.
They were closed today, so I'm going to go tomorrow to get a catappa leaf and ask for advice, and depending on what they suggest I might get some medicine too. Until then it at least seems like my betta isn't getting any worse, so he should be fine until tomorrow.