Okay, I'm at a loss- some bacterial infections have color loss, and grey patches..
Something is either up with his genes (as that isn't really a "normal" color for bettas.. at least how it looks in the pictures.
Do you see red streaks/lines? Septicemia can cause color loss, but unsure if to that degree.. and stress doesn't cause it to form that way, usually it's even over the body, or going from head to tail in intensity.
What I am looking at is the Rl, Red-loss gene, and how it affects other colors.
The red-loss trait will cause the red color in young bettas to disappear when it matures. When the fish ages or due to fin damage, the red color sometimes returns.
It is thought that the red-loss mutation was introduced together with the marble mutation, because most marbles do not show any red pigment . The red-loss trait is caused by distinct gene than the marble gene (which affects the black coloration), the red-loss (Rl) gene. The red-loss mutation is extremely variable in its expression and is dominant over all other Red genes, except extended red.
According to Dr. Gene Lucas, yellow colored bettas do not result from the action of a single gene. There is no such thing as a yellow gene that produces a yellow phenotype in Bettas. Yellow Betta are phenotypes. The yellow color itself was designated as 'non-red' by Lucas. This 'non-red' recessive gene caused bettas to be yellow where they would normally be red.
Okay, I'm at a loss here, and out of time- spent the last hour trying to get info, but not a lot out there with his symptoms.. I'll continue looking when I can. If it's just his genes and body going nuts, at least it's doing it in a unique kind of way.
I'd PM Darkmoon or OFL just in case they ran across this at some point.