Hi and welcome to the Chiari forum.'
I can suggest additional testing for ur DS....this is to see how his chiari is affecting his overall health....a CINE MRI to see if he has a CSF obstruction, and u also want MRI's of the thoracic and lumbar MRI as there r related issues like a syrinx and tethered cord and disk issues that can add to this condition,
With children it is important to also do a sleep study to rule out sleep apnea.....
Many with Chiari do have drop attacks but I have not heard of neone's lasting as long as ur sons have....so I really do not know what to tell u there, but it is possible for some sort of seizure activity as some have been dx'd....but u said that was ruled out....
U will also want a true Chiari specialist....we have a list for Canada and yes there r a few members from there...and here is a link to the list ...the list i snot a referral just a means for u to start ur research.
http://www.medhelp.org/health_pages/list?cid=186
Know that u r not alone, there are other parents going thru the same issues u r and I am sure u can help each other.....let us know how we can help.