Checking in again.
King's finrot appears to be gone and he is very happy in his new tank. I can't be sure, but his fins look fuller than they did, so I am assuming that his fins are beginning to grow back. (yay!) He has returned to his normal colors - no strange blue or gold tint. His dorsal fin spots are very hard to see now, though, which is strange. My best guess is it's a result of battling the fin rot. My fingers are crossed that they'll return as he continues to heal.
All of the stuff I ordered for him has finally arrived. He is now set up with one large plant, a betta hammock with the wire removed, a heater, a filter, and 4 marimo moss balls (all of which have been in quarantine for the last week and a half.)
I am using Stress Coat in his water and feeding him his new Spectrum betta food.
Here is his new home!
Thanks again to everyone for your help.