Can you please give me some advice on how to help him? I hate to see his grades going down when I know he could be doing much better.
Another idea that has worked for me with young boys I have counseled in my practice is to allow them to watch a TV show (this is always popular!) while they take quick written notes on the ongoing plot. They do not have to write complete sentences; this technique just encourages fast thinking and writing.
Secondly, having to think up the perfect idea may be slowing down your son as he gets started with his work. Talk with him and remind him to trust his "first idea" in writing. A good way to practice this is to have him try some writing "prompts". For example, have him write a brief paragraph answering questions like, "What is your favorite summer activity?" or "What was the best movie you ever saw?" Encourage him to be creative and have fun with this. The plan is to free-up his creative thinking and to help him trust his ideas. I hope you find this information helpful. Good Luck!