If you have the time to spend with him/her, give him/her responsibilities that require a lot of writing. You will need to spend lots of time in review/critique, however there's no better way to learn than by doing. Early in my banking career, I was given the opportunity to write sections of our press releases and annual reports; writing that got heavy scrutiny. As time went on, there was less and less red ink on my drafts until I became a fairly decent writer. I learned more from this than from all the English classes I took in grade school, high school, and college.