How many hens do you have? He's the new bird and he has no buddies, this is stressful for him. Mature hens can be really tough on a new roo, particularly a young cockerel. It would be kinder to integrate him slower to the girls. Use a wire crate or small separately fenced area for a while so they can all see each other, but not touch each other. Feed him and them on opposite sides of the wire, close together. Let them get to know each other that way first. Then when everyone seems calm, let him in with them for short, supervised visits, so you can see how it goes. It will take time, the amount of time depends on him and them, it varies with every bird, or group of birds, integration takes as long as it takes. He's immature, may have lots of hormones and little manners. He may be hiding in the nest box due to not being very welcomed by the girls, in some cases hens can really hurt a roo or even kill him, it just depends on the birds in question. Integration should always be slow, and after a quarantine period to ensure that a new bird is not bringing in any disease.
