I have thought about writing his written narrations down and having him copy them. And I have though about writing down tough words or words I know he will miss and he can use the list as a guide to help with spelling. The challenge for him is that he will most like miss easier words too. I am okay if everything isn't spelled correctly. He reads A LOT now and his spelling is getting somewhat better. Part of me thinks let it go and don't worry too much. My oldest had somewhat of the same problem for a different reason and she spells much better now. I just let her's go, wrote down the tough words and after a while there weren't nearly so many mistakes But my son's spelling is worse than hers was. For my son now, he does all of his writing in the DITHOR book with minimal help. I do spell things as he asks. I also spell stuff when he asked for help in the R&S book if he had to write a sentence on his own. With Bigger, we chose to do copy work when a choice was given for copywork or independent writing during the history notebooking pages. For science experiments, he usually wrote it himself and just would ask when he needed help with spelling. And with the dictionary entries it was hit or miss as to whether or not help copied something or came up with it alone.I know there is a bit more independent writing in Preparing. He will be writing more in science (questions and then the experiment). He will have the written narrations. He will have the sentences from the dictionary entries. DITHOR won't increase per se because he is continuing on with his book but it will have some independent writing.
So, if you had a child who struggled, how did you handle it as the writing increased? My hope is that because of the dictation and his increase in reading, spelling will just naturally increase and get better. We are just at the point right before that actually happens
Next, I'd have him look at the word and try jotting on markerboard or scratch paper to see if he could come up with 'what looked right.' Again, I watched for frustration and tried to maintain that balance of accountability and frustration. 
After about less than a minute of trying to spell it right on his own, I wrote it for him correctly spelled on the markerboard, and we moved on to another subject. As he has gotten older, I see him starting with using his history/science/etc. texts to help him spell words such as new vocabulary terms, people's names, events, etc., moving to jotting it in his HOD Student Notebook in pencil, reading it out loud and trying to fix it as he reads according to what he thinks 'looks right,' and/or putting it in his IPod webster dictionary to see if it's spelled right. Finally, as he reads it aloud to me, he reads it with pencil in hand and often fixes any few errors he still may have on his own as he reads it to me, and finally if there are any errors left, I jot them on a sticky note or a markerboard for him to fix as he's reading (if he didn't fix it already on his own). 
I just thought I'd share the progress I've seen through the years, and encourage you Nealewill that your little one will make progress too. Slow and steady, quality over quantity, teamwork over you're in this alone, and consistent patient correction win this race.