Hm... Ideally a specialist opinion would be best, but it sounds like that might be kind of unrealistic for you. On the frog forum I'm on, there are several vets that will do phone/email consultations for people (usually free of charge) who can't make it to a herp specialist, and then people usually take that info and recommendations to their regular vet. I don't know if there's a similar option for snakes though... Anyone on here know of any services like that that would apply here?
If you can convince your vet to aspirate some fluid and do a culture that might be the way to go... Even a typical small animal vet will have the equipment and knowhow to do that. Just tell him you're worried that the antibiotic isn't targeting whatever the problem is.
However, I realize that this sort of thing can get pretty pricey, and considering the circumstances no one would judge you if you had to make the decision to euthanize (though I'm still holding out hope he can recover!)
Is it just me or does that large blister look like it's deflating a bit and starting to crinkle up near the front? It's good that he's still actively seeking out water at any rate.