Writing skills, as measured by individually administered standardized tests (or functional assessments of writing skills) are substantially below those expected given a person's chronological age, measured intelligence, and age-appropriate education.
The disturbance significantly interferes with academic achievement or activities of daily living that require the composition of written texts (e.g., writing grammatically correct sentences and organized paragraphs).
If a sensory deficit is present, the difficulties in writing skills are in excess of those usually associated with it.
As you can see, the definition is really quite broad. As for the things you can do at home to help your son, the best would be to separate the role of "author" and "secretary." It's often very difficult to children to keep in mind all the mechanics of writing (spelling, capitalization, punctuation, etc.) at the same time they are trying to generate and organize ideas.
Encourage your son to begin with a prewriting activity to get his ideas down. This can be as simple as generating a list of ideas on small Post-its.
The second step is to move those ideas around within categories to begin to see how his paragraphs should be constructed.
Once he begins to write, he needs to come up with a topic sentence that links the ideas together.
During this first draft, your son should not think about spelling, punctuation, or anything else besides the structure and clarity of his writing.
After he has completed his first draft, your son should read it out loud to someone to see if it makes sense and if anything needs to be clarified or expanded -- teachers always want more details!
Finally, the role of the secretary comes in. Help him to check for run-on sentences, incomplete sentences, correct spelling, punctuation, grammar, and capitalization. Try setting up a cueing system at the beginning of lines where he hasn't caught his errors. For example, 2SP might indicate that there are two spelling errors in that line.
If he isn't doing it already, your son should be using a computer for most of his writing. This makes editing, revising, and spell checking much, much easier. In order to make the best use of the computer, your son should learn how to touch type. Diana Hanbury King's method for teaching touch-typing makes it much easier for most students. Once he knows the placement of the keys, then he can use a computer-based program like Typing Tutor to build speed. If he's not typing on a computer, your son can use a hand-held spelling tool like the Franklin Speller from Franklin Electronics.
Is your son receiving support services for writing at school? If so, try to engage the person delivering those services to help you to ask for accommodations in the classroom. Talk to your son's teachers about whether writing assignments can be reduced or if time limits can be extended. You can also ask whether it would be okay for you to take his dictation rather than having him write on his own if the act of composing remains a big problem. If his reading is okay, there are even some "speech-to-text" programs like Via Voice that will allow him to speak into the computer and then turn his words into print.
There are no quick solutions to written expression disorders. It will take time, patience, and hard work to tackle this problem. Consider having a professional tutor working with your son in this area. You can ask the school for a referral and then the two of you can work together to support him. Good luck!