Buy your child a journal and encourage him to write in it daily. Sitting down to write each day can provide your child with necessary writing practice, and writing in a personal journal will not make him feel like he's doing homework.
Teach your child to write in drafts. Many elementary students write until their assignment is completed, then move on. Showing your child how to read back through his writing and create a new and better draft will help him develop editorial and proofreading skills.
Work with your child to build his vocabulary. You can do this with flash cards or by learning one new word each day. The more words your child knows, the better he will be able to express himself in writing.
Encourage your child to read books instead of watching TV in his free time. Reading stimulates the areas of the mind from which writing originates and will help teach him grammar and vocabulary. It is also a good way to calm down after a hectic school day.