Sorry to hear he isn't well, but I'm glad you have a vets appointment booked 
Could be a salivary gland inflammation/injury. Could be a small piece of that wood slide, like a splinter, that has lodged in the soft part of the skin and is now inflamed/infected/fester....are you able to take a magnifying glass and get a good look at the center point of it?
If it is squishy (water bed wiggly, then likely not an abscess....