Like I said, I understand the urge to throw in meds, it comes from a caring place and wanting to save him, and meds are expensive! So I know it's because you care and want him to survive, and you're not alone, so many people have the same urge and do the same thing, so you're far from the first to do it. Heck, my fish cupboard has some medications I bought in a panic, but didn't wind up using, or only used once. Most have a few knocking around for the same reasons!
It's hard when a fish is sick, since we can't pop them in a carrier and take them to a vet. So we do the best we can with the info we have.
But bettas are probably the number one fish we get emergency threads about, often without being able to save the fish, I'm afraid. The ones from stores are so sickly, weak, prone to tumours, fin rot, those heavy fins dragging around, lethargy, dropsy- you name it. Sometimes that fish might look okay in the store, but is already doomed before you buy it, and if someone can keep a fish store betta alive for more than a year, then I consider that a success and they've been lucky, and done well.
When you say he won't respond to you, don't pester or touch him. Do the water changes on the other side of the tank from him and leave him be - that chemical soup won't be helping - just use a cup/jug/small container of some kind that isn't contaminated with anything to remove half of the water. Then top the tank back up with fresh dechlorinated and temperature matched water. Then repeat. There will still be some chemical traces in there, and running some carbon in the filter for a while will remove those, but those water changes alone will remove a lot. And again, fresh clean water is the best first aid, no matter what the issue is. You'll also need to run that carbon in the filter to remove the previous medications before adding anything else that you said is arriving today, or again, it'll be a chemical soup.
When did you get him? And the tetras? How long have they been in the same tank?
The rapid onset of two days isn't usual for fin rot. Sadly, he doesn't look good.
It might even be something internal that you cannot fix. I've never kept phantom tetra personally, but they're a bit too big for a ten gallon, really need a larger group, and some tetras can be nippy and bully other fish if not enough members of their own species. Then Bettas are fighting fish. Solitary and territorial, and I can easily imagine those fish crammed together fighting and nipping, even if you haven't witnessed it. Injuries from other fish to his fins and likely internally since he's fading so fast and onset was so rapid seems more likely to me.
If he does rally, then keeping him in the 2.5g alone would be much better and less stressful/risky for him, or any other betta. They're a fiercely territorial and solitary fish that don't like or need company. If he is passing away or can recover, the water changes are the first step either way. He'll be more comfortable not breathing and swimming in a chemical soup, and will either recover, or pass away in comfort at least.
No shade intended at all in my comments, BTW! Trying to be helpful, no blame aimed at you, just trying to help, I promise! I'm sorry you're experiencing the hardest part of the hobby already. You can have a successful tank, just need some advice and solid resources about stocking, like
@Naughts said, and we're here to help.
I hope he does recover, but even if he doesn't, please don't be too hard on yourself, or think about giving up the hobby. You clearly love your fish and want the best for them, have done a lot of research, and we've all made mistakes, there's a steep learning curve in this hobby. That's why forums like this exist! So we can help each other out and learn.