- an area rug
- the way you made the bed
- focal point
Your carpet is a little dull- I would add an area rug- the one at the top of the post is from Pottery Barn Teen. This will help the room to feel a little more finished and grounded.
Read this post on bed making. Your bedding is great- but the bed making is not. I know that 9 year old boys may not be the best at making the bed a specific way- but you can always try....
The big thing missing from your room is a focal point. In a bedroom- the focal point should always be the headboard wall. The area that theoretically would be the focal point is simply empty. An accent paint color is not the answer. You can do one of two things. Fist of all- you could hang wall decor above the bed that would act as your focal point- like the American Flag below that I found from Uttermost...
So - there you go- I would add a rug- re-make the bed and add a focal point. A few small changes- and the room will look fantastic. Another thought is to find a map that has better colors in it for the room- like this map from Boetti.