Jungle Fungus Eliminator is supposed to cover pretty much the most common bacterial and fungus problems. I haven't tried it, got some on order "just in case," but maybe you should try some when you get the money. It already contains salt. Jungle Fungus Clear is the same thing in tablet form without the salt. Neither are snail- or plant-safe.
But, yeah, I'd get the heater first and try some garlic juice in his tank.
It has a weak antimicrobial effect. I've been reading around, and the really knowledgeable people say you have to crush the garlic fresh for it to be effective, not the jarred stuff (I just read that today.)
Unfortunately Venus passed away yesterday morning...
I'm kinda sad but also I'm grateful that he lived long past what I thought he would. I knew soon would be his time, and even so, it feels so surreal that he's actually gone, when he's overcome so much before...
At least through his death you have learned that bettas will attempt to swim through any hole they think they can fit their nose through. Anyone else who has terra cotta pots--PLEASE fill the hole with silicon, a marble, or simply cut two circles of craft mesh, put one on each side of the hole, and then sew them together through the middle. You could also simply use pliers to enlarge the hole enough that the betta could swim through safely--file down any sharp edges of course.
Also, for any other/future bettas you might have--you should definitely invest in heaters. Adjustable heaters are not optional for tropical fish. Just because they can survive at room temperature does not mean they can thrive in it. If you order online you will save a lot of money, particularly if you buy multiple items at once.