Aww the poor thing. I bet they just tossed a live prey in there and walked away, leaving for extended times alone with him. I'd suggest you set him up on layers of soft, plain white papertowel...lots of layers so he can hide in them if he wants but nothing can stick to those sores. Hides, right temps, etc. of course. I'd likely put his enclosure in a walk-in closet or somewhere else that's dead quiet and has little foot traffic as the poor thing must be miserable and stressed.

Neosporin is a good idea, the one without the pain relief in it and buy the store brand from WalMart. It's just as good as the name brand and half the price. The only issue I'd be worried about hon is with that many untreated rodent bites there could be an infection brewing under the skin. I highly doubt this poor snake was kept in a clean enclosure either so put open, untreated bites together with a probably dirty substrate and I'd be very concerned about infection. It's probably wise to seek a vet's opinion on whether a course of antibiotics are in order for this snake to ensure nothing is going on that way. If you do see the vet make sure to ask whether the meds administered will affect digestion as some vets suggest not feeding until after the treatment (depends on the vet and on the medication).

Other than that, typical stuff....let him settle in quietly, no handling and so forth. I don't know about kingsnakes specifically but I know most milksnakes and cornsnakes will take f/t pretty easily. Just start with a smaller than normal prey item so he's not overly stressed by a big digestive thing, better he takes a few weeks or a month of smaller feeds that he can keep down than go to a larger feed and trigger a regurge I figure.

Remember all colubrids are escape artists at heart so lid clips are a super important part of keeping them. I'd suggest you take a pic of him now for your records so one day you can look back and see how far he's come. It's wonderful you've taken on this poor snake....good for you!