It's hard to tell from the pictures, but look closely at his old trouble spots. Is there any new growth that is perhaps clear or grey in color?
My betta Sashimi was adopted when he had pretty bad fin rot. Before I had it under control, he lost about half of his tail.
Now, 6 weeks later, it has mostly grown back. The new growth came as a grey, almost black color. It is only now beginning to lighten to the yellow of the rest of his fins.