That earlier photo with Santa...even then he didn't look real healthy to me, so that's not something he should have been "out & about" doing, but as a new snake
owner, you wouldn't have known or noticed anything- I'm assuming that photo was not taken in your home? It's adorable sure, but likely stressful and not likely at
the proper temperatures, either while there or during transport. Just not something we'd recommend doing, that's all. FYI, I've done programs with snakes for
years, taking them to a wide variety of venues: they only go if I've had them long enough to know they're healthy & they're normally transported in cloth bags that
are placed inside a cushioned styro-foam box or 'cooler' so they aren't exposed to fluctuating temps. going to & from. I've moved across country with them packed
this way too, never any health issues afterwards. I'm saying this only for your information, not to say you "caused" his illness, because that's probably NOT the case.

I'm glad you & your vet(s) are doing what you can, I hope you can get him back to healthy. Did they just take a guess as to what antibiotic to administer? or was
lab work done to make sure the medication chosen will be the most effective? This matters a lot, as antibiotics aren't harmless: they kill off the friendly gut bacteria*
& are a bit rough on the snake's body, so it's not something you want your vet to guess on, even though lab work takes time & usually a lot more money. If an
"educated guess" is taken & it proves to be ineffective, it means you've put your snake thru injections for nothing (stress, & hard on the body) while time was lost not
getting it right, & then it will have to be repeated with the "right" medication. (*assuming your snake recovers, it's a good idea AFTER a snake has been on antibiotic
treatments, to add digestive enzymes to their food to help out, just like we take "probiotics" the same way. They do make them FOR reptiles, & that's all you should
use, because their gut flora is not the same as ours or that of other pets.)