Looks fungal to me. Treat with Seachem sulfaplex.
The meds you've been using outside of Furan 2 are snake oil products IMO
Change out anywhere between 25-50% of the water daily when you aren't medicating or between medication doses. If you go 2 or 3 rounds on the meds give him a few days of water changes in between to rest his organs from the medication.
You might also try Seachem Polyguard and rotate it out with sulfaplex.
Try your best but be prepared to euthanize. He's in real bad shape.