Good JOB!
I just rescued a betta from walmart a week or two ago as well. He's a gorgeous red white and blue marble, and was in the dirtiest cup with a live fish there, I honestly didnt think he'd make it, but I got him home, acclimated him VERY SLOWLY, added some AQ salt to help with stress coat building since I was out of stress coat, and now he's probably the most interesting betta in my room. He's definitely the most curious
Hopefully things will turn up for your little guy and he turns out the same!