I've been pretty busy last couple days, just caught up with this thread, but unfortunately you've already heard from others what I'm about to say...nothing new.

Antibiotic injections will not cure an abscess like that appears to be...it needs surgical intervention by experienced herp vet which I truly hope you find, and soon.
That a vet expected this to "drain" surprises me- snakes have pus that's pretty solid, so any substantial abscess like this can only be fixed with surgery- it won't drain.
Less likely but also possible is some kind of tumor, but I'd put money on "abscess".

It's amazing your snake still eats...that has to be painful and it pains me to see & think about. There really isn't much we can do by forum other than reassure you
of what you're seeing & point you in the right direction...it's not something you alone can cure. So sorry, poor snake.