Gritsar, you get yourself into some situations. Some talent you have. I've never had NN's and would never have thought of that as a potential problem. After seeing what my former Speckled Sussex roo did to the feathers on the back of the head of some pullets when he was young, I can appreciate the problem. I assume the injury is up where a roo normally grabs a hen, up on the head more than down on the neck and pretty well centered on the back of the head. I've seen some pretty vicious pecking order pecks from hens toward the head and neck, but if the injuries are consistently in the same place, yeah, I'd certainly think roo.
Why have you ruled out Ollie? In other posts you said he was trying but was too big to complete the task. He would have the size and strength to grab hold and maybe the frustration to try too hard. Just a thought to help confuse you.
The way I understand your set-up, Thor and Impy occasionally meet each other and don't really fight. I think they are sort of in the same territory at least part of the time but keep out of each others way. They sleep in separate coops. OK, I'll admit, I really don't understand your set-up. Anyway, I've found that roosters raised together, even in a father-son type of arrangement, often find an accommodation as the younger one matures that does not involve a whole lot of real serious fighting. When the young one matures enough to really challenge the established rooster, there can and probably will be a fight or two, but even then they often reach an accommodation, even if it is a switch in dominance roles. If you separate Impy until he grows up, the fighting when he is reintroduced to the flock is likely to be more severe. It is still possible they will reach an accommodation, but it is likely to be more severe getting there. It is also possible that they will have serious fights if Impy continues to be raised with the flock and is not separated. From what you've said in other posts, I think Impy will eventually become flock master, dethroning Thor, whichever way you go. That does not necessarily mean he will kill or seriously injure Thor, just that he will eventually win.
You asked for other options. I'll surprise you and mention one that does not involve the kitchen. It is not something I'd do and I don't think you would either, but you could debeak Impy, cut off a short bit of his upper beak so he cannot grab hold so viciously.
Why have you ruled out Ollie? In other posts you said he was trying but was too big to complete the task. He would have the size and strength to grab hold and maybe the frustration to try too hard. Just a thought to help confuse you.
The way I understand your set-up, Thor and Impy occasionally meet each other and don't really fight. I think they are sort of in the same territory at least part of the time but keep out of each others way. They sleep in separate coops. OK, I'll admit, I really don't understand your set-up. Anyway, I've found that roosters raised together, even in a father-son type of arrangement, often find an accommodation as the younger one matures that does not involve a whole lot of real serious fighting. When the young one matures enough to really challenge the established rooster, there can and probably will be a fight or two, but even then they often reach an accommodation, even if it is a switch in dominance roles. If you separate Impy until he grows up, the fighting when he is reintroduced to the flock is likely to be more severe. It is still possible they will reach an accommodation, but it is likely to be more severe getting there. It is also possible that they will have serious fights if Impy continues to be raised with the flock and is not separated. From what you've said in other posts, I think Impy will eventually become flock master, dethroning Thor, whichever way you go. That does not necessarily mean he will kill or seriously injure Thor, just that he will eventually win.
You asked for other options. I'll surprise you and mention one that does not involve the kitchen. It is not something I'd do and I don't think you would either, but you could debeak Impy, cut off a short bit of his upper beak so he cannot grab hold so viciously.