My parents were foster parents to 40 kids, most teens. I fostered for 4 years, mostly little ones, but three teens. Just so ya know where I'm coming from.
First, go slow. These are a lot of changes to adjust to, for both of you. If you feel overwhelmed at any time, just imagine how he must feel, with os many fewer coping skills.
I loved homeschooling and highly recommend it. However, this is a unique situation. Most homeschoolers join groups, like for science, or they homeschool for religous reasons. SOme join groups for taking fun and educational field trips. This way, the kids meet other kids in the same situation. You could try "finding your tribe" here at MDC to see if there are any homeschoolers in your area. Some areas have 'schools' kids can go to for just a class a week or so. Another way to meet kids. If your DN is a really social kid, he'll manage. If not, you have to. I would, under the circumstances, consider public school.
'House rules' are helpful. Write down, say 10, of the most important things you need your dn to know/do. Post it on the fridge. THis worked great for my mom. SHe said it was hard to argue with a list on the fridge! Your's might look like this;
1)Clean room or no 'puter time. Clean room means dirty clothes in hamper, dirty dishes in dishwasher, trash in basket, clean clothes in drawers. (Don't expect perfection. for the first few weeks, accentuate the postitive. If he has books under his bed and his blankets are on the floor, but the dishes are taken care of and his laundry is ok, just concentrate on what's done.)
2)no name calling or yelling.
Be sure toi othink about what things are really most important to you. Change the list as needed, but don't just keep adding to it. Pick 10 things, work on them. Really 3 things would be plenty to work on, but you'll probably need to make some things clear.
What behaviors do you see as immature? IT's hard, not having his background. Many of these immature bahaviors are probably coping skills, assuming he's ging to be living with you for a reason. He probably has areas that are delayed, because of whatever the background is, and will work through them in time.
I hihgly recommend the GD forum. SOrry, I don't know if you have kids, or any experience, so please don't be offended. I am a jump-in-with-both-feet kinda person, and you sound like you may be too. If you are new to 11 yo's, I'd go much slower than you may beplanning. Eleven and moving to a new home is tough.
I'd homeschol for just an hour or so a day, take many, many field trips. He'll be new to the area, so everywhere makes a good trip. I, personally, feel unschooling to be a lot of work if it's going to be done right, and think it's easier to simply teach for an hour or so, and then concentrate on learning with im the rest of the time. Go to museums, not just the big ones. Search on-line and visit some of those tiny ones. Childrens' museums are still great at 11. Call your local school and see if they have any clubs or activities open to homeschoolers. SOme do.
Call the YMCA, a great way to meet others. He can join teams, swim, hang with other kids at the 'clubhouse' most have, and you can join him for activities, too.
If you go to kid friendly places and hang out a lot, it will help. I mean, places where it's ok to be with an adult, but you might meet other kids. (we have Kidsport, playgrounds, soccer fields. If you go kick a ball around, sometimes another kid will join in, and you can fade out.
Inviting others over to dinner, as someone suggested, is a great idea. Be blunt with them. Tell them you are trying to help your nephew get adjusted and don't worry if it goes badly. Just serve dessert, take notes for the next time, and try again. (Is your nephew tough or sweet? Is being 'cool' all important? DOes he play a musical instrument? Love soccer?) It will helk you to figure out who to invite over.
And I know you didn't ask, BUT... heck it's late, I'm on my way to bed, and I have nothin g to lose by sticking my nosy neck into YOUR life, ...I'd consider letting him earn video game time.
I know, tv is evil and so are those games.
But they're a great way to connect with a kid his age. Boys can't sit down, face-to-face like many girls, and talk about how things are going. In the car, driving down the highway, focused on the open road, they'll sometimes talk. But video games often have the same effect. IF you were to allow, say only Mario and Zelda-type games (mazes, strategy, shooting magical creatures with sling-shots, etc. and race games. I highly recommend 'Monkey Ball' for silly fun.) and you 'got sucked in' with him, you'd share something. For an eleven yo boy to have an aunt who loves to challenge him to a game, well, it's even better than a parent.
Run to the library, get...heck, I forget the right name, Raising Sons? someone here will know. I've found guiding to be better than forbidding, even to dangerous stuff, sometimes.
Boys are soooo different from girls. I'd like to get ahold of those 'Free to be, You and me' folks and then there was a book I had....Raising Baby X? If you didn't know is the baby was a girl or a boy, how differently we might raise them.. All bulll, IMO, cause boys are boys, and girls are girls.
But I digress.
Best of luck to you both! Let us know how it goes.