Or he may have injured it and was the cause for the 'sick' chick,What I realised is that, the cockerel has obviously been mounting this chick a fair few times before I noticed a problem. Perhaps the cockerel senses something with this chick but, then I would assume a different type of aggression verses mounting?
or the chick got hurt some how and he is now wanting to drive it out of the flock?
No 'good' cockerel would mount a 9 week old chick.
It's really hard to say what went on in this situation, especially if they range all day.
Best of cLuck fingering it out.