Sadly many vet have limited hands on experience with dealing with snakes, mouth abscesses are often due to a broken tooth or in some cases a prey that bites the inside of the mouth or another type of injury and can develop months after an the original problem issue. Considering the extent surgery is most likely to be your next step to remove the caseous pus followed by anti-biotic treatment.
Here is how you can find a Herp vet in your area http://thereptilereport.com/category/vet-directory/ - https://arav.site-ym.com/search/custom.asp?id=3661