I was considering if the issue came from the neighboring flock, or the wild birds, it could be helpful to isolate your flock from contact (if at all feasible) and/or take down birdfeeders for now.He is a strong rooster! I live in a forested town area with a large wild bird population. The nearest flock is a stone's throw.
Lots of great help and advice from Wyorp, as always

Hoping you'll get this nipped in your flock early and won't be playing vet much longer!