Find a different vet NOW! A RI should not last 3 months. If Baytril didn't work those two times, your vet will need to try a different course of antibiotics. Did your vet swab your snake for a culture to see what kind of RI it is, and what antibiotics would in fact fight and kill it off? Please state where you live at, I'm sure someone on here can point you in the direction of a very good herp vet.
Also I take it, your snake is either popping, wheezing, or has bubbles coming out of his mouth/nose. Letting him soak in a tub of warm water for 15-20 minutes will help him loosen up some of that mucus and let him breath a bit better.