The other roo might be able to sense a weakness in him. You mentioned he's small right now and can feel his breast bone easily. This may be an underlying problem separate from the feather loss.
It might be a nutrient deficiency caused by parasites. Do any of the other birds seem thinner than normal? Its possible that they could all have worms, but this little fellow just has an excess load of them. I don't think nutrient deficiency alone can cause feather loss, but it can induce self feather picking for sure.
Weight loss combined with self picking may provoke the lower roo, or even hens, to pick on him, causing the loss of head and neck feathers.
Maybe you could try worming them all and see if that helps.
It might be a nutrient deficiency caused by parasites. Do any of the other birds seem thinner than normal? Its possible that they could all have worms, but this little fellow just has an excess load of them. I don't think nutrient deficiency alone can cause feather loss, but it can induce self feather picking for sure.
Weight loss combined with self picking may provoke the lower roo, or even hens, to pick on him, causing the loss of head and neck feathers.
Maybe you could try worming them all and see if that helps.