One more thought: IF you're not within travelling distance* of a qualified herp vet, remember that "tele-health" can work for pets & vets too. The idea is to see a local vet you trust, even if they aren't reptile specialists (which you've already done) but have them contact a recommended herp specialist (for a fee, obviously) to get their diagnosis & recommended treatment that the local vet can help you with. Your local vet can show them pics & lab work, then get their input as to proper treatment plan.
*I also know that the middle of winter isn't a great time to be on a long road trip with a sick snake- a distance that might have been "do-able" in another season may well be off the table now.
I'm impressed that at least you sought vet help for your snake, but just not quite the right way so far. All the best, let us know how it goes? Please do not wait- the faster you get this snake on the right antibiotic, the better chance he has.