The other possibility is that his working memory may be average but he has better language based skills. So once his reading became automatic he was fine but spelling is taking a bit longer. As long as he is not behind (and it really does not sound like he is), I would give it another year working diligently on those skills. Deficits do not always show up in bright younger children. The standards to have average achievement scores on standardized tests are relatively low and often weaknesses at his age really do work themselves out. Unless the problems are extreme I would be hesitant to evaluate a younger child.