It could be down to a lack of space combined with hens coming into lay and trying to secure their territory? For some reason he has lost their respect as a defender and food finder. You will certainly need to keep him apart until totally healed as hens will peck scabs off. We had one case where two hens ate half the cockerel's comb whilst we were out. Once they had tasted his blood even 3 months of isolation hadn't broken the habit. Stockholm tar is what we put on minor injuries to stop hens pecking, but it needs re-applying every two days. If you can identify the cannibals Bumpa beak bits may work?
As MF says, re-introduce him with lowest pecking order hens. Ideally the hens should be introduced to his territory, not theirs. Once he has established his small flock add individual hens and wait until he has trodden them before adding the next. Give him treats he can call his hens to - it works well for us.