Depending on the school, the RPI scores may or may not be useful. If the school generally teaches above national norms, they won't be an appropriate tool. However, it does look like it might motivate a discussion with the school for a curriculum-based assessment.
The Iowa Acceleration Scale is a good tool to determine the appropriateness of an acceleration. There are a lot of factors to consider, and test scores are a starting point. That being said, I would, however, first have a discussion with the classroom teacher about how she sees your son and his growth and performance in the classroom.
The best way to enrich a child in reading is to read and to discuss books. If the child doesn't like to read too much on his own, consider books on tape when riding in the car.
