Sometimes a child is a good reader but not a good speller because he is reading so quickly he doesn’t take the time to look at the words’ spellings as he reads. Prepared dictation can help establish that skill as a habit, because the first step in dictation is to read the passage and look for any words he doesn’t know how to spell. If he practices that skill every week, it should help over the long haul.
Using Language Well does not put emphasis on spelling rules because Charlotte didn’t. With all the exceptions to the rules in the English language, they can become more of a hindrance than a help.
That being said, if you think the rules are important to this child’s success, you could introduce applicable rules as you come across words in his dictation exercises that he needs to learn. If a particular rule will help him learn a particular word, go ahead and see if it helps.