Purely from an interior design POV, I recommend finding a warm beigey colour or a neutral bluey colour (with a hint of slate) for the walls. Don't buy any coloured furniture. As other people have said, ANYTHING cool right now will be babyish and tragic in a year or two. He can put his personality on it with accessories.
Definitely get a loft bed with something underneath, either a desk, storage or a sofa. Something like this would be awesome:
https://www.familywindow.co.uk/products/stompa-casa-c-high-sleeper-with-sofa-bed-desk-shelf If there's not enough height, a "captain's bed" has drawers underneath so at least you're partially doubling up there.
I'd recommend getting one big bit of storage furniture rather than lots of little bits, like a shelving unit that goes from floor to ceiling. It'll create fewer crevices to clean between bits of furniture, use vertical space, and be more versatile becaus eyou can organise big shelves differently more easily than lots of little shelves and drawers. Always look for a small, simple footprint and as tall as possible. For any piece of furniture you're thinking of getting, google "X with storage" to get more bang for your buck from the footprint. Provide many MANY clear plastic boxes or neutral-colours boxes/baskets to contain stuff.
Definitely get an over-the-door shoe organiser. They can be used to store loads of stuff, not just shoes!
For versatile decor, buy a couple of big, plain picture frames - A3 is a good size. Buy a few cheap posters online and it'll be easy to swap them out when they get uncool. Also, buy a HUGE corkboard for him to cover with ever-changing crap. I'd let him pick the duvet cover but not the curtains/blinds, because it's easy and cheap to swap the former and a PITA to swap the latter. Get a darker version of the wall colour, maybe with a stripe, but nothing too patterned. I also highly recommend hanging some coat hooks on the wall so he has no excuse to leave "But I'll wear it again, honest!" clothes on the floor.
Consider getting the book "Make Your House Do The Housework" by Don Aslett. It is very good on storage space that's easy to keep clean.
And as an ex-tween...
As for reading, at that age my parents took us to the library every week or two and their policy was that we had to take out the maximum number of books (eight, IIRC, so not a crazy number!) every time. We didn't have to read them, but we had to choose them, check them out, keep track of them, and return them on time. Once we'd gone through all that hassle, most of the time I felt I might as well read them...! I think it's more important to focus on the habit of reading than what he's reading. My parents still bang on at me to read Great Literature, but to be honest I'm just not that interested in novels. However, even though I lost it for a few years around university, I still have the habit of having a book on the go and read widely in nonfiction and biography as well as some "trashy" detective novels. I don't think I'm a lesser human because I don't want to read Thackerey and Tolstoy.
Figure out a policy on mess and cleanliness in his room vs privacy and ownership, especially if you're using some of the storage space. I remember being hugely impressed by a friend of mine who said she let her primary school aged children draw on the walls of their bedrooms. They were only allowed to do it in pencil so she could paint over it but she said she didn't have to look at it in there and it meant they didn't draw on the rest of the house. I thought that was a very sensible approach! (She owned the house, obviously...) Is it important that he keeps him room
tidy, or just hygienic (e.g. no food left in there to mould, hoover once a month kind of thing)? Or does it really
need to be decently tidy so you can get to your stuff? Or do you
want it to be tidy to teach him to be a human enough to have sanctions if it isn't?
Why does his room need to be tidy? It'll help you decide where to draw your battle lines when it becomes an issue.
I think a good question for encouraging people of all ages off screens is "What would you like to learn how to do?" and then provide the materials to facilitate that. You might have to engage in a bit of, uh, Socratic dialogue to find something that isn't "Do Minecraft better", but he'll be more invested in it if it comes from him.
Also, if you're wanting to limit screens then I think it would be nice if you can to frame it as something you're doing together. "Son, I think we both need to cut down our screen usage, so from now on the Wifi will be off between X and Y times.* We can do something together then if you like or you could do [other thing] and I'm going to do [good example thing]."
*The interwebs can tell you how to make this happen.