Swim bladder
Has there been any change to the tilting angle when he's not swimming? Is he still active and trying to move around or has he become lethargic?
Because of his high stress level and the fact that you've seen him poo since he started to fast. I'd say it's okay to give him (only) one or two peas when you see this in the morning... then nothing else. Still wait. To curb his hunger slightly, might help settle him down so he's not struggling so much looking for food. Just don't feed too much to further aggravate things. Watch for signs later that he poos normally and let us know if he ate them or not.
Fin rot/bacterial infection
This sets in either from poor water quality (which you've improved for him since bringing him home), from stress (which is obvious), or from bacterial infection (which you already know and are treating for now with medication). So you're doing the right things for that new development already. Same thing for bacterial treatment.
Stress
This seems to be a large factor in everything here, which is not unusual. First, he's in a strange place... it takes real effort for him to move around because of the angle he's at. If you don't get the plants right away, put in anything you have that's aquarium safe so he can hide, relax, and rest. You could use smooth rocks, bottles, a coffee mug.. add the plants for more cover once you get them. For this tank, I'd sort of clutter up one side with items he can tuck himself in between so he feels protected... leave the other side of the tank open so he can venture out if that's what he wants to do. This lets him find his own comfort areas to hang out in.
The salt you were adding is not an irritant to him, that also serves as a tonic that will help calm him down. It doesn't need to be discontinued when medication is added.
The key to all this, of course, is going to be time. You've removed all the negatives you can control to give him the best chance to recover. You're doing all the positive things needed to help that along, too. The rest is now to wait and see. He's either going to get better now or he's not. Accept that so you're prepared whichever way this goes.
Let us know how he's doing in the morning... and remember, they often seem to be getting worse when, in fact, they really are getting better.