Interactive writing is the easiest way to jump-start a reluctant writer or help a beginning writer interact in the writing process. It's simple, requires little prep, if any, and no special materials.
You can customize the process to your needs and make it fun, even including multiple children in the process.
Just as you would encourage your child to read with fluency, you should encourage and nurture fluent writing. This means allowing him to write freely without stopping to spellcheck every word. Teach your child that spelling, grammar, and word choice can be worked on later. The most important thing is to get their ideas flowing and onto paper.