I'm guessing writing is not something he's particularly excited about/interested in? I'm not aware of any specific writing problem associated with ADHD. Both my husband and son are ADHD and neither have any particular problem with writing...
I would focus on making sure he's learned the basics - summarizing, identifying key events, outlining, etc. Also, I would see if he works better with a distraction, or not. For some odd reason, my husband actually focuses better with a tv on in the background.
