Writing skills come along with reading skills. If a child reads alot discuss the book with them. If you find that your student expresses himself very well verbally, sometimes they lose their train of thought because they cannot get it down on paper fast enough. You might try a small tape recorder, "Dragon" self typing computer software program, or other device that will help him to hold his train of thought.
However, if descriptive expression is what you are looking for, then I suggest you ask your student to write about something very simple, may a special tree in your back yard or on your block....then ask your student to describe it in its environment, the grandeur of its massive branches, leaf formation, etc.
I thinking reading more books will be the best way to teach your student to express himself better, and taking time to talk to him/her about it.
E.