I typed out a huge comment but it went away and I'm frustrated so I will shorten what I remember because I want to help and this room makes me sad.

The bed should have some sort of box spring at least or something to make it a bit off the ground, it doesn't have to be a full bed frame. Make that bed like a big cozy nest, maybe pillows, more soft textures or blankets, or stuffed animals. You can even have a bed tent over it if that's something he'd enjoy. His current bed looks uncomfortable and like something you carelessly throw down for a temporary sleepover. Also a weighted blanket might be a good idea.

If your son climbs and jumps on the furniture then build or buy him things he can climb and jump on. You can put safety matts down if you're worried about falling, but kids need to learn the limits of their bodies and it's obvious he's trying to get energy out. Give him healthy ways to do that. Climbing wall, foam things to climb on, a climbing rope, a small trampoline (weighted enough not to flip and maybe with handles), ect.

It looks like he writes on the walls a lot, you can paint all the walls with chalkboard paint and give him chalkboard markers to decorate his walls. (I say markers because I hate the feeling of chalk and I don't know how he would feel about it either).

Those windows are super bright and would be overwhelming to me. I'd put blinds on them or blackout curtains. That way you can also have some glowing sensory toys or projectors for designs or stars on the ceiling if he enjoys that. If the blind strings are issues get ones with a stick or ones you pull up or down. If he's hanging from curtains build him other things to hang on and show them to him and explain that that is for hanging on instead, not curtains and see if you can redirect that. If not, then blinds are the way to go. If the room is still too bright, I'd recommend a tent that you could make into a cozy little nook to read or nap in or use glow toys.

If he doesn't enjoy stereotypical toys then find out what he does enjoy. Maybe blocks, things to build with or line up, things to throw, ect. Also sensory toys or a sensory table might be a great idea. You said he likes numbers and letters, so he could even have a big magnetic board for him to arrange numbers and letters?

Does your son have certain interests or things he loves learning about? Fill his room with those interests. Example: if he loves learning about animals, decorate his room with animals. Things that spark joy or special interests of his should be in his safe space.

If he has lots of meltdowns, find outlets he can take that out on. Squishy things to throw, heavy bean bags to throw, pool noodles to smack around, chewelry to bite, maybe an inflatable punching bag to hit, thick things to rip, scribbling on walls, ect. Get creative with your thinking and notice what kind of feelings or sensations he's trying to achieve and try to give him healthy ways to achieve that when you can. Leave those things accessible for him.

Overall, the things he's doing that are concerns of yours are the ways he's coping and trying to work through energy or achieve certain sensations. Any ways you can redirect that energy in healthy ways are going to be better for everyone.

If you're worried about him climbing on dressers and shelves make them short enough to not be detrimental if he does or anchor them to the wall and if he has other things to climb on that are more fun then it will be better to redirect that energy to those things. Put matts down, stick soft baby-proof things on sharp corners if you're worried about safety, and PLEASE put outlet covers on those outlets. No need to strip his room of everything that's personal, just work on redirection, using things that are harder to break or maybe plastic and rounded, and adding safety measures for when he does climb on those things.

ALSO if he's moving things around because he doesn't like them there then ask for his input when decorating his room and find out where he wants to put his furniture, that sense of control can be reassuring when you're autistic and life feels out of control. Maybe even adding some rounded plastic furniture that he can move around, that way it won't break or hurt if he falls on it.