Open mouthed breathing is not at all a good sign...taking him to the vet....as you are, (yay) is a wonderful idea and will probably yield better results. My boyfriend bought an 8ft male BCI to breed with my female...he had a little bit of wheezing a few days later, followed by occasional open mouthed breathing two weeks later. I told my boyfriend he was an idiot for buying a snake without me present so I could give it a health checkover,as he is very unexperienced with snakes, but he has learned his lesson....I told him the snake needed to see a vet, cause it was seriously ill, and was probably sick when he bought it....it died less than a week later. Hope yours gets better!