You say he is young, just how young? His age and especially maturity will make a difference. How old are the hens?
One mile away is probably enough so he doesn’t try to go home, but it would probably be a good idea to keep him locked up for several days, maybe even as long as a week, to make sure he doesn’t go looking for home when you let him out. Usually that’s not necessary since you have hens, but with living animals you never know. I’d be more concerned with this if he is an immature cockerel with mature hens compared to a mature rooster.
If he is a mature rooster and your hens or pullets are laying eggs, he should start mating with the hens immediately and make the flock his. Mature roosters are about as easy as it gets. An immature cockerel may be bullied by mature hens until he matures enough to assert his dominance. I’ve had 5 month old cockerels earn the respect of mature hens and just take over a flock, though that; really rare. I’ve had an 11 month old cockerel still being bullied by the head hen, though most of the other hens had accepted him.
It’s not purely an age thing for him, it’s how fast he matures. And some of that depends on the personalities of the hens. It’s pretty common to blame everything on the male but the females have an important part to play in this too. Each individual chicken has its own personality and each flock has its own unique dynamics. When you talk about behaviors of any living animal you don’t get guarantees.
With a rooster I know is mature I’d generally say just turn him loose, even with home a mile away. That should be enough. With a cockerel it can get a bit more complicated.
