My Serafino has the exact same thing going on! It has been that way for a couple of months; I've tried antibiotics, Melafix, IAL extract, salt, fungus meds, parasite meds.... nothing has made it go away or even get the slightest bit better, but his health doesn't seem to be failing him or anything so I have no clue what it is
I've noticed the same thing on my bettas, it might be loose scales, they've never gotten sick from it. It probably happens when a betta accidentally scratches itself and lossens a scale.
It could very well be old age, that's what I was thinking about Serafino, he's easily 3 years old by now. I don't think it's loss of scales though, as one of Serafino's spots runs into his eye... if he has scratched himself there he'd have more than missing scales to worry about
All the spots are on his head, is it the same with your fish?