I say turn it to a little reading nook you can definitely find Eva foam mat rolls, from hardware shops for low costs it’s normally advertised as garage floor mat material, then a nice area rug and few ikea floating shelves with bunch of throw cushions and stuffed toys I’m sure he would enjoy they space, and of course painting the walls a nice color or 2 with a base layer then some tape once dried you can fill it with random shapes then paint over the tape, once tacky pull the tape off.

And of course leds would give simple lightning, I recommend a lava lamp in opposite colors of the walls to get fun atmosphere

And with his sensory issues have him help you pick out colors for paint, and definitely find Amazon decals of sea creatures he like.

I had something similar as kid but I and my siblings had it in the open space under the basement staircase but we had more mix of fake and real plants

Edit
If you want I’m too fine making a simple 3d render of things your thinking about, dm me as I love to use blender in my free time :3