I would not try fogging him with anything, and especially not eucalyptus oil, which could irritate the respiratory tract even more. Right now, focus on setting him up correctly: ambient temps 82-84 (a little higher than the normal 80-82, since he's under the weather) humidity around 65%. If your tub is large enough & you can offer a small basking spot that won't affect the overall ambient temp in the enclosure, you can try offering a hot spot of 86-88 degrees or so & see if he makes use of it.
Without seeing pics of this snake and knowing how snotty/bubbly/gurgly he is, it's hard to determine the extent of his respiratory issue, especially considering that bloods are very vocal and huffy to start with. Your vet should at least perform cultures & sensitivity tests to determine what medications are appropriate here...baytril is ok in some situations but there are other medications that may be more appropriate, depending on what is causing the infection (if there is one).