That's an infection plain and simple. Identifying the culprit is essential because many bacteria are resistant to certain drugs. You really need a vet that can perform a debridement ( the medical removal of dead, damaged, or infected tissue to improve the healing potential of the remaining healthy tissue) and have the dead material tested in a lab to identify the culprit. Only then can they know what antibiotic to use, and then, the antibiotic needs to be in the snake's bloodstream, either given orally or by injection.

If that can't be accomplished right away, (and it wouldn't be cheap, so cost can be a factor) if it were me, I would do a warm soak as best I can, and rake off the dead material myself and keep applying a strong antiseptic to the area. If crust builds up again, remove it. You want to treat the healthy tissue underneath. But in the end, fighting it topically might not be enough if the infection is too deep in the tissues.

Like Steve suggested, you might give Hibiclens a try (or any skin cleanser containing a few percent chlorhexidine gluconate) and apply several times a day, don't rinse. Bacteria which have developed resistance to iodine or certain treatments often have less resistance to chlorhexidine gluconate because it's use is relatively new. You've probably heard of MRSA. It's a strain of staph that has developed resistance to normal treatments/antibiotics. Even iodine won't get rid of it from an active skin infection, but chlorhexidine gluconate will. Just make sure you continue treatment a couple of weeks beyond apparent healing, to prevent creating a resistant strain of the bacteria.

At this point, you already tried iodine but didn't get rid of it. So, next time you try it, it's probably not going to work at all if the bacteria have become resistant. You need something strong, but something that isn't iodine. chlorhexidine gluconate fits the bill but you gotta get that dead tissue off so the medication can bond with the live healthy tissue underneath.

If you took him to a vet and followed his instructions or had him treated, I would file a complaint. It has gotten much worse. If it was done right early when you first showed us...