Try turning your boy's wall into a high-speed road. You can make it into a raceway by painting an oval, or make a series of intersecting rows with black lines along with intersecting lines. Create the medians by adding thin white dotted lines. Use sticky putty that is used for hanging posters to attach lightweight matchbox cars to the road to make it look a little more realistic.
Another way to decorate the walls is to add decals. This can be done by covering the wall with a sheet or by purchasing decals that attach safely to walls without peeling paint. Some ideas on the walls can include superheroes, villains, favorite sports teams logos or players or scary insects and bugs. In addition to these items, sports pennants can be used.
Choose lighting fixtures that fit your boy's preference. Purchase lamps that are sports- or character-themed and which match the decorations you placed on his wall. You can also hang decorations from the lights. For something really unusual in your boy's room, replace the standard white light bulbs in his room with lights of different colors.
Continue with the theme you have chosen for your child's flooring and bedding. You can buy rugs from specialty stores or on the Internet that match your theme. For the car theme, you could purchase one that looks like a racetrack. For sports-themed rooms, you can get one that is the logo of your child's favorite team. Rugs can also be purchased in the shapes of animals and superheroes as well.
Bedding sets can be purchased as a "bed in a bag" kit. These include covers, comforters, sheets and pillow covers that all match the same theme. These can be purchased for car themes, sports themes, superheroes or villains.