I've seen the pattern/color change in a past betta. Yours looks like he has white patches coming in but it's not fuzz, it's just his scales changing color. Would you say that is an accurate description of what's happening to your guy?
With my betta, he had started out the same color as yours but he was a Crowntail. He started to get patches of scales turning white/losing their color and I couldn't find anything that would cause whitening scales but when I asked around at various pet stores and forums, the consensus was that it was probably a bacterial problem. So, I treated him with API's E.M. Erythromycin. After 48 hours of treatment and 16-year-old me praying constantly that he'd get better, his color came back and he looked great! He was eating great and being normal again. That lasted about 2 weeks. One morning I woke up and he had gone from looking like the perfect boy he was to almost completely white overnight. He was panting on the bottom of the tank, his fins looked like garbage, and he looked like he was miserable. I desperately tried to help him by treating him with Erythromycin again but it didn't help him at all the second time. I could not find any disease online that could do this to a fish and to this day I can only guess that it was a bacteria that went dormant after the first treatment until it was strong enough to kill him. He died less than 24 hours after I tried to treat him again.