Oh dear, that is troubling, poor thing!
How do you have your T5 mounted in his tank? Is it mounted on top of the screen top
or below/underneath of the screen? How far away is the UVB from him?
Could he have fallen somehow & hurt his back or tail?
What is his supplementation schedule like on his calcium intake?
Let us know how he is doing.
Awe, poor little guy. It does look like a calcium deficiency/MBD. Exactly what T5 UVB light do you have? (Brand, wattage, length)
How long have you owned him, have you had him his entire life? And has he always been under this UVB light, or did you just switch him over to it?
I don't think it's an impaction if he's pooping regularly, plus he's definitely very thin, and that's usually a sign of inadequate lighting...
You mention he's very thirsty, what are the temperatures of his 3 temperature zones inside his tank? Have you checked them since this started? And what type of thermometer are you using to take the temps? I'd actually do this immediately if you haven't yet tested the temperatures, because if they spiked for some reason they may be in the lethal temperature range...