For Spelling Wisdom and Using Language Well, I would start him at the beginning of Book 1. Do not worry about “catching him up.” There is no “behind,” just where he is. Teach your child, not the curriculum. 🙂
When a child starts to write his narrations, there is a transition period. Expect his written narration to be shorter than what he is able to give orally, because now he must not only think about what he wants to tell, but also how to form the letters, how to punctuate, and a host of other writing mechanicals. For written narrations, focus on him getting his thoughts on paper for now. Some narrations will be longer, some shorter. And that is okay! There is no set rule for how long a written narration needs to be other than long enough to tell what was learned.
This article on the length of narrations from our Narration Q&A series in our Learning Library may be helpful to you.