I’ve struggled with this question a bit, because I face the same issue with my students. Other than providing a “word of the day” to introduce them to new words or telling them to use a thesaurus, I believe the only real way for them to expand their vocabulary is for them to read books. This seems like a lost art with the current generation sometimes, but improved skills in reading, writing and speaking all come from reading. It doesn’t matter what it is, as long as it’s written properly (meaning no social media, etc).
Sorry I can’t be more help with this issue. This would be more of an individual solution for each person.
Teaching In Real Life 