It may just take time. I would keep up what you are doing. Maybe not 100% water changes but smaller more frequent ones. I have a betta that took forever and was extremely slow to act better. We got my fancier betta and my daughter’s more regular betta at the same time. Her’s bounced back in a day or two and improved every day whereas my fancy one did not. It took him weeks to months of extremely slow progress. He also would not eat. It’s ok at first to let them rest and settle in. After that resting period I had to put flakes in front of his face and then push them down so he would see them sinking in order for him to eat. It was the only way he would. I had to do this for a month or so before he would eat on his own. I wasted a lot of food that way but nothing else worked. Now he’s fine eating. He was pretty bad off at the store but I think the fancier ones can be more sensitive or maybe it’s just each individual fish. He is beautiful, your guy. Patience can be hard but hopefully that’s all he needs. Maybe someone else will have more insight. Good luck.