I have a 10 year old with dysgraphia, so I can totally relate to your quandary.
Can you get accommodations for the dysgraphia if he is put into a higher class - i.e. typing his papers or dictating them vs. writing? Will there be a lot of in-class composition or will most of the assignments be take-home where he can work at his own pace?
For my own son - and for his two older siblings who didn't have disabilities - our choice has always been the harder classes that challenged them to their limits. I'd rather my child get a C in an AP or advanced class than breeze by with an A in one that doesn't really force growth. That being said, you know your kid and know whether the additional challenge of the higher placement will be more destructive to his confidence than the trade-off of more challenging content.
I know Microsoft Word can analyze your writing and tell you the grade level of your writing (as a former reporter, I used to check that mine was at about a 6th grade level to meet the needs of the publications I wrote for), but it's not good at telling you if you were revisiting a topic too often or using a phrase repeatedly. I found with my kids that showing works here much better than anything I could find online - I would take a paragraph they'd written and rewrite it for them and then ask them to follow suit with the rest of their paper. Being able to mimic a cleaner, more direct style was beneficial.
Hope this helps.
