Do you think the narrator makes any discoveries at the end of the story as he cradles his brothers body?
The brother only knows that he is frustrated at having to always wait and slow down for Doodle who can't keep up. He is childish and selfish as children are, but that doesn't mean he loves his brother any less. He is truly sorrowful when Doodle has passed, but in his limited knowledge, he is unable to connect the symbolism until he has gained experience and insight that adulthood brings.
This story is so effective, I think, because it is told from the narrator's retrospective point of view. He exists in the story as two people--the boy who played a major role in his little brother's death and the adult, with adult understanding, who looks back on his own conduct. I think mrsmonica is right on target in drawing a distinction between what the narrator understood at the time and what he understands later. As an adult, he comes to recognize not only Doodle's precious nature, but also the "knot of cruelty" that existed in his own nature.
I think that Brother is overcome with grief and remorse at the end of the story. He feels guilty over always having wished that Doodle could be normal and do all the things that average boys could do. He is grief-stricken at the knowledge that Doodle is gone forever and that he (Brother) has contributed to Doodle's death. As far as discoveries, I don't believe that Brother truly understands the symbolism of the scarlet ibis until he is an adult remembering the episode.
Brother is very regretful over his actions and is extremely sorrowful, although what the scarlet ibis symbolizes is not known to him at the time.