Was the suspected bacterial cultured? If not, get it cultured. No vet should prescribe a long term antibiotic without culturing the infection (if it's bacterial).
Also, RIs can be viral, fungal or even as a symptom of an underlying more serious disease.
In other words, unless you've had a culture done, you're leaving a lot of stones unturned.
If your vet hasn't considered viral or other potential causes, you're not covering all your bases.
...and it's time to see a new vet.
If you decide it's not worth the expense to correctly get to the bottom of this, then do your proper duty by your snake and have him properly euthanized. That does not entail sticking him in the freezer.