I believe it all depends on mindset. If he thinks he's bad at writing, he won't write better. I'm not sure whether talking to him would help, but you can always try. But, in my opinion the best way to improve writing is to read more. Also, you can try doing some writing exercises with him. And to prove that he is improving, for example you can run it through editing software or grade his paper (if interested, check out essay grading at http://essay-grader.com). Seeing that you are actually improving is a good way to change his mind set and show that he is a good writer, and the only thing he needs to do is just to push himself just a little bit more.
The smartest apples are the ones you don't leave last to eat. A smart person understands writing is important. Follow your writing let it be who you write as. As a good writer you must understand that you need to explain your writing to make sense. Making sense is being happy with the explanation and evaluation of your writing good or graded unfair. If writing sentences is about articulating words, then write the words that are least interesting to the writer and more interesting to the writer for best results.