Ah, the beauty of homeschooling, which allows the freedom to tailor the program to fit the student!
Because nothing inspires confidence and the will to go on so much as success, I would suggest that you begin with Language of God, Book C. Beginning with lower level coursework will provide the opportunity to review concepts that may be a little shaky, and reveal educational gaps. [Your son should be able to move fairly quickly through Book C.]
As he builds on his knowledge of grammar, your son can move on to Language of God, Books D and E.
In addition, as your son progresses through these levels, be sure to have him apply what he is learning to his writing. For example, if his recent grammar lessons focus on proper nouns and quotations, he may wish to do a bit of creative writing using [and identifying] proper nouns and quotations.
May God bless and guide your homeschooling adventure!