I spent a summer doing summer camps for a museum and one of the kids' favorite activities was making murals. We'd buy a huge roll of butcher paper and tape 2 lengths together (so it's 6-8 ft high, and as long as it takes to cover one wall) and then help your son draw out an outline of a scene (beach, city, farm, mountain, etc) in chunky markers. Then tape that monster to one wall in your son's room. He can then draw, color, cut out, and tape on elements to add to the mural whenever he's feeling creative. For example, if it was a mountain scene, he could make a paper waterfall to add, draw and cut out forest animals, skiers, sparkly snow, 3d paper campers and tents, etc.
Then photograph and replace the mural whenever your son needs a change.

This way he gets to decorate his room with his paper creations and be creative, but it's a bit more focused than the paper strewn all over the floor.