Wooohooo, reading teacher to the rescue!
Alright, Iconoclast. It's good that you understand that children can be more developed in various subjects than others. And often times students who excel in math and science struggle with language arts and vice versa. Students who are into math and science are more left brained whereas language is a more right-brained domain.
So, what is the MOST IMPORTANT thing you can do to help your son write better? He needs to read more. U might think I'm crazy, but reading and writing are connected. Kids who read more are more likely to have stronger vocabulary and better grammar because they pick up the author's style of writing when they read books. And then they try to emulate the author's writing style.
Also, keep in mind that everyone has their own style of writing. AND, there are different types of writing. A letter to your boss is going to written differently from a letter to your friend.....which will be written differently from a research paper on reptiles.....which will be written differently from an autobiography......which will be written differently from a persuasive essay. See what I mean? Usually kids learn in school how to write for different occasions.
To help your son write better, you want to point out positive things that GOOD WRITERS do when they want to grab the reader's attention. How does a good writer grab the reader's attention? Well, maybe the author starts his essay with a** shocking statistic** or sentence. Maybe the writer captures your attention with irony or humor. Maybe the author is capturing your attention with sound-words, STRONG VERBS, or detailed descriptions, clarity, and punctuation. Point these things out....and encourage him to incorporate only one of these strategies one at a time.
IF your son struggles with vocabulary/spelling....then according to educational research, vocabulary/spelling is best learned through reading books. Teaching kids vocab through an exercise book is not very helpful because it's not a relaxed and authentic setting.
And the last thing I will mention is maturity. Trust me....I've taught preschool, elem, and middle school.....writing skills develop as kids grow older. And maybe the teacher didn't say anything because she feels he is doing well for a 3rd grade level. But try the tips above.
For one week, you might want to focus on writing with proper punctuation. The following week you might want to focus on writing clearly by avoiding run-on sentences. The next week, you might want to focus on using strong verbs....or using irony....or a particular attention-grabbing writing style.