I know you probably don't want to have to wait a super long time but if it were me I would wait until he was about the same size as my hens or close. Since they both are still growing and roosters tend to be slower growers, that could take a while. But I would definitely still even just ho!d him around them and just let him kind of get used to them, or just kind of let him sit on your lap so if he feels comfortable enough after a while he might jump down and join the hens. I definitely like to let the roo kind of get used to my hens some so I'm not throwing him in to a band of scary, crazy strangers, lol. How many pullets do you have?