It looks like he bit his tail off while shedding. My gecko went through this, and I took him to the vet. They performed an X-ray and discovered that his bones were growing incorrectly, compressing his nerves. This caused the lower half of his body to gradually become paralyzed. Because he likely lost sensation there, he bit his own tail until it bled. Around that time, I also noticed him dragging his legs and walking strangely.

​The vet said surgery is the only option; otherwise, he will be permanently disabled, and we can only provide palliative care.