1. Si is a measure of verbal reasoning. It does depend a little on vocabulary, but, as you note, his vocab is good. One typical hypothesis would be that he has good concrete verbal skills (verbally-acquired knowledge and basic skills), but not as strong abstract verbal skills (problem-solving, inference). The additional info you've provided does raise some questions. If his spelling seems inconsistent with his reading skills, it may be that there is some kind of learning challenge, especially since the error you give as an example is NOT a phonetic equivalent. It suggests that he is not accurately distinguishing the vowel sounds.
2. Yes. This is one aspect of what is called cognitive efficiency. Good for getting lots of work done, but not necessarily reflective of deep or original thinking.
3. I think I would seriously consider this. In the absence of academic reasons for concern, I might think of the discrepancy as more of a learning preference for visual-spatial/nonverbal tasks--hence a tilted or skewed, but not disabled, profile. With your additional information on some discordant presentation with regard to his reading, vocabulary, and spelling, I would wonder a bit about areas that might require remediation.
