A simple solution is to use a “COPS” sheet. Cops is an acronym for Capitalization, Overall appearance, Punctuation, and Spelling. Since the majority of the population, regardless of ability, are visual learners, a poster that includes a picture really helps. You can get a poster here, and a checklist here.
When your child is finished writing, whether the assignment is a sentence, a paragraph, or an essay, having him self-monitor by using a COPS checklist is helpful. It breaks down the task of editing so that he is only looking for one thing at a time to fix.
1. Capitalization: are the beginnings of all sentences capitalized? Are proper names and places capitalized? Are there any capitals that don’t belong?
2. Overall Appearance: do the sentences make sense? Is there space between the words? Is it legible?
3. Punctuation: is the proper punctuation used at the end of each sentence? Is any additional punctuation where it belongs?
4. Spelling: are the words spelled correctly?
Basically, your child will be reading over his work four times, looking for one specific thing each time. I have used this for years with students of all ages, and it works well. Of course, your child may not find every error in his work, but self-editing will make him more mindful of his writing.