A big part of your goal with a grade skip vs. subject acceleration with age peers is to get intellectual peers, other kids who are working on the same things he is and who have some of the same interests.
Having your child feeling lonely and weird and isolated sitting alone in the back of the class on the computer or constantly being pulled out to work one-on-one for subject acceleration is the sort of thing that you want to avoid.
Explicit and effective supports for organization, time management, assignment tracking, etc., to compensate for disabilities involving weak executive functioning, like ADHD, is certainly part of what you would want to get in an IEP as your child moves up anyway, but it would be doubly important to ensure that they were there when expectations were for the average level of functioning of an older child.
Have you filled out the Iowa Acceleration Scale? It might be helpful.
