Choose some bedding you know he'll love. Then choose paint and maybe you could do a mural on 1 wall. A rug for kids sitting on floor & beanbag chair(s)

Murals Lots on wayfair.com

With such a statement wall, plus all the things he will have in the room, I’d keep the bedding solid like the bed just showing on the left in this photo. Too much pattern can feel chaotic in a small space.

I’d measure the two larger walls and try bed under the window to see how much useable space can be taken advantage of. You could also create an on line room planner with room’s dimensions/structural components to test ideas. Head to Ikea and provide those dimensions to a consultant. Explain his needs: a surface for lego building/storage, arts/crafts, which can double time as a desk. The right configuration can accommodate his changing needs.

Regarding a rug, wait to see the size needed based on furniture size and location. Decor wise, again wait and base on furniture selected/wall mural and with him being ten, he may have some preferences.

We did shelves for our kids for stuff they wanted to display . Our grandson had transformers and tons of them we did the same type shelving for those .I find boys do not need the same type of closet space as girls so why not utilize part of the closet for some nice bins for his lego . Let him choose his bedding, keep the wall color neutral. I think you could do a long table under the shelving since he is now 10 he can reach much higher . . A to scale floor plan showing all windows doorways closet would be ahuge help here . Post on graph paper in jpeg format here in a comment DO NOT start another post.

We put a shelf that ran the length of the wall where his bed is. She was an athlete so it gave her a spot to put her trophies, ribbons, game balls, dolls, etc. Her bed was under this shelf.

I second going to IKEA if one near by and get a desk or stacking units for storage.

Consider an architectural drafting table instead of a conventional desk. One that can be lifted, lowered, tilted, etc. And a stool instead of a chair. Far more versatile. And, perhaps, a wall mounted light that can be moved to focus on his work.

If the bed can fit another way, we might suggest trying it. Perhaps in the middle of the window? If not, then leave it where it is and look at adding a desk, shelves, or even a built-in shelf around the bed for storage. Below are some pictures from various designs of ours to help with more ideas:

Summit Townhome · More Info

Artists Haven · More Info

Willoughby Way · More Info

Willoughby Way · More Info