He's not a rooster yet, he's still an immature cockerel. That complicates it, a mature rooster would be less unsure as to what would probably happen.
Several different things might happen. One is that the integration might go extremely easily. If you have enough room it often does. You generally just read about the horror stories on here. The older girls might beat the crap out of him, especially the dominant hen. I'm not sure what state his hormones are in, those typically kick in around 15 weeks but can start as early as 12 weeks or maybe not until after 5 months. When his hormones kick in he may try to force mate the hens. They may fight back or may just run from him. If they run he may let them go or he may chase them down and force mate them. A mature rooster should not bother those young pullets until they are ready to lay eggs. By that time they are ready to mate. I don't know how an immature cockerel will treat them.
I don't know how many coops and runs you have, how they are tied together, or where they now sleep. Some people would just turn him loose with the flock and see what happens. If you have a lot of room that often works. With my set-up I'd probably try something like that. If you free range one risk is that he will not join the others but will run off or maybe sleep in a tree. You can try housing him in an enclosure where they can all see him for a week before you turn him loose. That might help the others to accept him and could teach him a good place to sleep. In any case, observe what happens and base your actions on what you see.
I had one cockerel become flock master to a flock of mature hens and immature pullets at five months. I've had one take close to a year to do that. Most of my cockerels take over the flock around 7 months but that can really vary. Some of that depends on his personality but the personality of the hens, especially the dominant hen, has a lot to do with that too. I've never had one take over a year to take over the flock but I don't doubt that some people have.
I imagine when they free range, your two different age groups probably don't hang together. Mine don't, they form sub-flocks by age and stay separate. Some can even form cliques that branch off even more. I would not expect one rooster won't keep them all together. I don't have the faith that a lot of people on here do that a rooster provides that much protection. They can be a lookout to warn of danger, usually this applies to something flying overhead. Many dominant hens do pretty good at that too. If something suspicious is going on many roosters will put themselves between the flock and whatever that is to check it out so there is some protection there. But in the predator attacks I've had the rooster has never been scratched or lost a feather. Once a real danger is identified mine lead their flock to safety instead of fighting a rear guard action so they can escape.
I personally would not have a rooster just for protection but many people on this forum do. If you decide on a rooster I'd want a mature one past the hormonal stage of a cockerel. Then you could just turn him loose with the flock and expect very little, if any, drama.
