Don't worry he's got the marbling gene so he's changing color, its not sickness just a genetic fun-time (never know what those marbles will do for color/pattern!). If he was sick he's be losing all the color in his body, the red band on his head/back it a tell that its not sickness. Amusingly I have a betta that looks a bit similar your current shot but he's got the marble gene and is slowly getting more red instead of less.thank you
Though, I'm worried my red Betta, He went from a solid red to what he looks like in the photo now.
I'm not sure if he's just changing color or he's getting sick.