Back from the vet! He thinks the cause is definitely an injury of some sort, not bacterial or fungal in nature. And as of yet, he doesn't think an infection has set in because of the injury. The end of the tail is still alive and flexible, and responds to being touched. Because the white area appears pinched, and the tail doesn't bend fluidly in this area, the theories are that he either bit himself or pinched it on something in his tank. Since I caught him attacking a dish last night, I wouldn't be surprised at all if he did bite it himself. The vet doesn't think the tail is broken. However, he said it's STILL possible that he can lose the end of the tail, but right now it's not at the point where amputating it is necessary; since it's still alive, he thinks I have a chance to save it if I do a little physical therapy on it, by getting the blood flowing by applying warmth and gently rubbing it. So my instructions are to rub it every night and watch VERY closely for signs that the end of the tail is losing condition. If this happens, I have to bring him back...and... snip snip.
He says it's possible he'll always have the little "bald spot." PING IS SO WEIRD.

He's definitely kept me on my toes.
We weighed him again -- 27 grams heavier than last Wednesday.
Also, I saw the vet at a different office than last time, and it's $25 cheaper for a basic office visit than the animal hospital.
EDIT: This skink is so incredibly cute too. You'd think he'd want to hide after yet another stressful vet visit. And at first, he did. Then I started boiling some chicken, the room started to smell like food, and he came out of hiding and has been staring at me ever since.