wow it been a month. I love updated like that!!! You doing such a good job!!! Try to feed him with frozen food. People saying it has a lot of protein in it and very good for the immune system. One of my betta that i have already a little bit longer than 2 years ,and i decided to feed all my bettas with frozen blood warms. I never did it before since i have bettas, about 7 years. But back to that particular betta, he never was sick always acting healthy and eating good,but somehow look like sick. His fins looked like they clumped. But since i feed him with frozen blood worms i really think he grew bigger and his tail not clumped any more . I do it once a week though,because it pain for me . You need to keep it in the freezer ,and just take a little piece of it , thaw and feed. It also pretty messy so i do it when i do full water change. You just try to give him one blood warm at the time so it don't sink to the bottom and stay there, and contaminate the tank. I am using a tweezers to feed them. But i would really recommend it, unless you already doing it. I want also try frozen daphnia, i didn't try it yet.
Also not sure if you doing it, make sure you rinse/swish the filter media in the tank water weekly with every water change.
And also i don't remember if i gave you the link from Oldfishlady , i am going to post it for you. I just like her recommendations:)