Sounds like a bad case of neglect Dan. Best to just let the anger go though and focus on getting John better. Stable husbandry, regular feeding (don't rush that, remember this snake hasn't been digesting properly so too much food too quickly can do more harm than good), little handling, etc. is the way to go in my opinion. Make sure the humidity is good and he'll likely shed soon with the proper food in him now. Soak him if needed to help that old shed come off cleanly. Get a fecal done as soon as you can just to make sure all is well that way and make absolutely sure you fully quarantine him. Only care for his needs after you've done any other herps in the house and scrub afterwards. Personally we keep all snakes in quarantine on pure white papertowels to watch for mites and also for the ease of collecting fecals and monitoring healthy defecations and urinations. He's likely dehydrated so access to a constant supply of clean water is a must of course.

We had one snake come in at 18 months that was barely bigger than our 8 month old. With proper prey volume and a consistent feeding schedule she sprouted, literally gaining before our eyes in both weight and length but doing so in a steady, healthy manner.

It's wonderful you've taken on this male. Get some pics now so you can look back one day when he's a big ole healthy boy and see the great changes you've helped him through.