I assume you can hang items on the walls and are you thinking of a particular theme? What color are the walls at present?
There is a variety of wallies available... or soft wall decor ... picture frames... can you put shelves up? If so, maybe paint them to match the bedding or the rest of his furniture...another idea...hang GOSSAMER fabric, comes in a variety of colors and themes...make it a feature behind the bed...or you could hang it as panels on the wall, with a few picture frames, also painted to match the decor, added inbetween the panels.