I do not know about giving antibiotics to snakes specifically, because so far I've been lucky enough to not need that, and I'm certainly not a vet...however from what I know about antibiotics in general, it is not a good sign that you are more than 1/2 way through the medicine and your snake is still showing lots of signs of illness. Is there any improvement at all? I would recommend you call your vet and give him an update.
Also, do you know if the vet did a culture? This would reveal exactly what nasties your snake is infected with, and which antibiotics are most effective against it. If your snake happens to be infected with a strain of bacteria that is resistant to the antibiotic that was prescribed, then there is no surprise that you aren't seeing lots of improvement. I would ask the vet to do a culture if they have not already.