I'm so sorry, I was afraid it was something like this (a tumor). If he was my snake, honestly, I would opt for euthanasia now so he's not suffering any more. I'm so very sorry.
And not to second-guess your vet, but in snakes, oral antibiotics are generally not effective because snakes are cold-blooded & digestion is not reliable- that is, they don't eat on a regular basis, & even less (usually not at all) when they're ill, and antibiotics must be given in a way that levels remain constant in the bloodstream to fight infection- thus, in snakes, antibiotics are generally injected. Either way, they wouldn't have helped a tumor though.