You know what? I'd bet anything that he also picked up some nasty cleaning chemicals off the floor! I especially suspect that strongly since he got so much better with a second water change.
I'd personally do some more small changes when he's stablilzed a bit more and add something like StressCoat to help him recover his protective slimecoat. In this way it helps guard against bacterial and fungal infections. Watch closely for signs of infection all over. Don't use Bettafix or anything else with melaleuca in it.
Thanks everyone! It's been a couple of weeks and it seems like Loki is as healthier and fiestier as ever haha!
Thinking back, it may have been the constant 100% water changes. I read only recently that 100% is bad, and at most 50% should be done. I'm thinking it was a combination of a weird chemical that could have gotten on him when he jumped out, plus maybe unstable PH in the new water.