You are describing more than 20% weight loss, sluggishness, and some sort of visible injury. Did the vet's office understand this is not for a check-up, this is a possibly seriously ill animal? You didn't say how fast this weight loss has occurred, but I would say it is possible the snake won't be alive by the time of the appointment next week. I'd recommend calling them back and insisting on an appointment much sooner than that. Most (I would think all) vets reserve some of their time for urgent situations so they can see those animals that need it the very same day, so I very seriously doubt the vet is truly fully booked until next week, they are just fully booked in those appointments they will give out to less than urgent situations.