Home details do not exist in a vacuum. All items in the room need to be considered. Please give us a floor plan, measurements, and pictures of each side of the room. Thank you.

If it's within your budget, I might have a corner cabinet built to provide shelving for storage above and fake doors on the bottom to disguise the fact that there is no storage below (unless there is a way to open the lower wall and have some use of that space (what is inside the wall there?).

For a less expensive option, you might be able to find a corner cabinet with those dimensions that could sit on the top (or be wall mounted) and provide some Lego storage or display (with glass doors to eliminate dusting Lego, ugh!)

On the order of these:

Add two triangular shelves and let him fill them up with things he loves. And when he gets older, the very back corner of the shelves will be a great place for him to stash thing he doesn't want you to find!

I would remove the piece of furniture in front, attach Lego baseplates to the entire face of the nook, and let your son start building 3D structures that extend from the panels and into the room. Allow his creativity to go wild. I did something similar when my kid was that age. He is now a teen and still has remnants of Legos in his room.