Ah...a missing piece.
If his brother was culled last week, the cockerel is under a lot of stress (missing his brother, trying to prove his value, scared, etc).
There's always an adjustment period after flock membership changes.
Please give him some time to settle down. He's missing his brother and scared he may just disappear too, if he can't prove himself.
I must have missed this - but what is the goal with the breeding? To sell the chicks? For the eggs? For the meat?
I have two goals - eggs and meat. So it's not going to matter too much to me what the new chicks are. They are either egg layers or meat.