You have to catch scale rot and/or blister disease early (before it goes septic) and I treat it not only with dry clean conditions but also up the temperature for a while (up to 90 degrees) and dab on some povidone iodine a couple times a day for a few days. This will usually clear it up within a week or two. The damaged scales will mostly remain.

If it's already systemic, and that is, internal infection in the blood, your snake needs a vet and antibiotics to have any chance of surviving it. That doesn't look like it's fungal to me. It's more likely a bacterial infection and it looks like it went quite deep but also has healed some. It looks to me as if it were a case of blister disease more than scale rot. If conditions are too damp/dirty, little non-septic pockets of fluid form under the skin. If it goes on too long, the fluid gets infected with bacteria which then can spread to the blood.

This is 100% preventable and is caused by bad husbandry. You must keep garters bedding dry and clean.

I really think if your snake is lethargic and not eating, he probably is fighting an internal infection in the blood and needs antibiotics. Even then, it may or may not be too late for that.

It looks like the pockets of fluid have already drained and that's a good thing. Good luck. I hope he makes it.