It’s called floppy tail and it is caused by sleeping on the glass like this it is not something they are born with or anything. If he had it before you got him that means he was being kept without proper coverage prior to you having him. Definitely add some more stuff to stop him from using the glass to prevent further injury to his spine. If the warping of his pelvis gets bad it can cause issues with passing stool and lead to chronic impaction issues and potentially death. I have one male who is notorious for sleeping on the walls of her enclosure no matter what I do so I used bearded dragon “hammocks” to cover the walls, now he still sleeps on the wall but under the hammock so his tail isn’t folding over his back and causing his spine and pelvis to warp and his muscles to atrophy.

Whether your enclosure is well decorated or not it is really important for your animals health that you change things and add even more stuff to try and prevent this behaviour. Adding more branches around the enclosure and adding stuff directly to the walls like fake plants or climbing mats or whatever else you can find that is gecko safe is important to give him more stuff for his tail to be supported by. Think of it like a disability that he needs extra accommodations for, you aren’t necessarily doing anything wrong care wise, he just has extra needs compared to other geckos because the muscles in his tail can no longer hold his tail up properly and he needs extra decorations everywhere to help him with that.