Some things arn't adding up. Puss in reptiles is not something that will flow out of an opening. It's caseous or cheesy. This could have been serum, saliva or some other idiopathic fluid.

Most things, be it an abscess, cyst or tumour don't appear overnight. An easy way to dismiss this was that it went unnoticed until now. If it did truely happen overnight, it might lead me to think of an injury hematoma or blockage of a salivary duct ect. The first three are still possibilities too.

As per the veterinarian. I might think he's seen this type of problem many times before, and thus dismissed it as a tumour as all the outcomes are all the same. Or perhaps he's it's an educated assumption based upon mammilian experience. Either way, without knowing the vet and being there to discuss his thoughts, it's pointless to guess why he suggested what he did. All I can offer is what procedures I might think are appropriate, me NOT being a vet.

For any unknown lump, the first sensible cause for action might be to find out what it is. Everything from fine needle aspirates, tissue biopsies, cultures, and imagery are available to veterinarians. With this information, I would add it to how it is affecting the snake (discussed history, examination, behavior, bloodwork) to come up with likely differental diagnostics and a treatment plan as needed.

Now my way out there guess on what I think it is without even seeing the snake... my money is on a salivary gland tumour. They are more common than many people think.

I hope all turns out well for you and your snake.

Ian