but not sure how badly he will get beat up - or killed - if I let him free range with the others. Any ideas/advice? Thanks in advance!
You have two different things working, a single chick that happens to be male. You want to integrate him with a flock of mature hens only, no other males. I don't know what your coop or other areas look like but they free range during the day. How badly will he get beat up or killed if he free ranges with the hens? No one can tell you. Sometimes it goes extremely smoothly and you wonder what all the fuss was about. Sometimes it can be deadly.
It is harder to integrate a single chick. Typically with several chicks they quickly learn to avoid the adults until they mature enough to join the flock. Until then they form a sub-flock. But they are social animals and like to be around other chickens. If there is only one they tend to try to hang with the adults. Some adult hens are more tolerant of that than others. Some single chicks seem to need close contact more than others. This doesn't matter if it is a male or female, single chicks can be harder. People do this all the time. Since yours free range you have a huge advantage over people that keep them contained in small pens. As always, the more room they have the better.
He is a boy. at nine weeks the hens probably don't know that or care, but he will soon enter puberty. Reading on this forum you can get the opinion that all mature hens go out of their way to beat up or kill any boy that shows up. It doesn't really work that way. Some hens will go out of their way to beat up on any chick, boy or girl. Most don't. Some do seem to hate boys worse than pullets, but some of that may be explained by behaviors.
Once the cockerel hits puberty his behaviors start to change. His hormones tell him to dominate the flock so he tries to mate the hens or peck them. It's called a pecking order for a reason, setting it up often involves pecking or even more violence even in a flock of all females. The mating is not really about sex, the one on bottom is accepting the dominance of the one on top, either willingly or by force. With an immature cockerel it's typically by force. Very few mature hens will willingly accept the dominance of a snotty teenage brat. Often they run away, sometimes they fight. Your flock has a dominant hen that probably wants to stay dominant. If she allows him to mate another hen she is allowing him to be dominant. So she knocks him off and maybe chases him to beat him up. With plenty of room he should get away, but if you had them in small pens it could be a different story. At some point he will mature enough to stand up to her and take over. That may mean a day or two of serious fighting between them or it may go really smoothly. His size doesn't matter that much. He may be a lot larger than her and she still dominates him or he may take over at a smaller size. It's more about personality (his and hers) and maturity than size.
So what do you do? I'm not sure what all your facilities look like so i'll be generic. I'd house him in a separate coop/pen for a week or more until he starts thinking of it as home. It needs to be where the hens can see him, at least during the day. This pen might be in the coop or totally separated. After a week or more, when I could be around to observe, I'd open the gate or door and let him free range with the hens. Base what yo do after that on what you see. If he is getting beat up to the point you consider it dangerous lock him back up and try again in a week.
I don't know what will happen. What I'd expect is some initial curiosity from the hens and he will probably try to join them. They will likely peck him to keep him away if he invades their personal space. Give it some time to settle as long as he runs away and gets away. He will probably hang as close to them as they will let him and soon learn how close that can be.
At night he might go back to his "coop" and sleep there. He may join them in their coop. I'd let him decide. If he does go into their coop they probably won't let him sleep on the main roosts with them. As long as it is not in the nests and is predator-safe I don't care where mine sleep. When he matures enough he will move onto the main roost with them. With cockerels who knows when that will be. I've had some that could manage that at 5 months. I've had one that took 11 months before the head hen accepted him.
When the hormones hit him, things will likely get exciting down there. He will start trying o dominate them. Some might be OK with that but it usually involves a lot of chasing and running away and forced matings. Some people find that period really hard to watch, some of us accept that as part of the maturing process and natural as long as no one is injured. It's often harder on the person watching than the chickens themselves but it is violent. Injuries can happen. You may want to lock him up until he matures enough to take over more on his personality and magnificence than him resorting to violence. With mine that's typically around seven months, though it can be sooner or later. The personality of the hens, especially the dominant hen, can have a big effect on that.
I'll repeat this because I think it is important. Sometimes this goes so smoothly you wonder what all the worry and fuss was about. Sometimes it ends badly. You don't get guarantees with the behaviors of living animals. Also, go more by what you see than what people like me on the internet say that may keep them in totally different conditions and likely have chickens with different personalities.
Good luck, however you decide to proceed.