Glad he is doing well.
I have noticed that trauma in my flock can result in white feathers. When Amazing Grace had her wing torn off -- she had some strange white stripes during subsequent growth. If a chick went through trauma or illness -- I saw more white -- or even wider bars. Kind of makes sense since the default feathers are white -- it could be that the pigment doesn't get added during the growth time when the chicken is fighting to live, or to regain health.
I noticed this happens in my older birds a lot, especially ones that pick fights or are at the bottom rung. There is one hen who always gets one white feather in her otherwise dark crest.