It looks like scale damage. Sometimes one of my bettas will lose some scales when he rubs up against the driftwood in the tank.
Another one of my bettas has recently been a picky eater. This may be because I'm constantly changing their diets up to try and include the broadest range of nutrition, but after completely changing his water and deep cleaning his substrate due to some microbial life I noticed, he's doing great and eating anything I throw at him.
So long story short, could be he's just being picky, or his water quality is bothering him.