Instead of paint...
Have him pick out POSTERS for the walls (skate & snowboard posters tend to be fairly brightly colored), plus some other fav things.
You can paint a white or babyish bookshelves orange (or green, or blue).
A few street signs on the walls.
A brightly colored beanbag type chair.
Match a couple extra pillow cases with the bookshelves (orange, green, or blue).
Hang a first aid kit w/ a big red cross on the wall (full of band-aids, neosporin, etc.). He'll need it for the various scrapes cement riders need! (Can you tell I'm biased, I'm a surfer/snowboarder ;)
SINGLE COLOR curtains. Trust me. My son's 9... and the next couple years any kind of multicolor thing will give you a headache. Littlekid stuff all "blends". Big kid /tween stuff tends to get fairly garish (esp with the 80's theme that's back in vogue). You'll be grateful for that neutral background and a limited number of colors over the next few years!!!