Hi and welcome to the Chiari forum.
First, ignore the verbiage on the MRI report and wait to see what Dr Rosner has to say...there is not clear criteria that all use that is updated for dx'ing chiari so the term means little to nothing to me...it may refer to the length so it may be 5mm and that is what many Drs use to determine if one has chiari, but chiari is the malformation of the skull, not the herniation, that is the result of the small skull...and the symptoms and how it is affecting ones health is more the issue, if there is a CSF obstruction, if there r related conditions....
We do have a list of questions for the NS in the Health Pages....
http://www.medhelp.org/health_pages/list?cid=186
Chiari is not something that much time is devoted to in med school, so too many Drs go by what they were taught and in many cases it is out dated info that they were shown, a true chiari specialist is doing his/her own research on chiari...if u see a few diff Drs u will see what I mean.
Make sure they check him for the CSF obstruction, sleep apnea, tethered cord, disk issues, ehlers-danlos,ICP.....
Keep in mind not all with chiari r surgical candidates and sometimes a NS will opt to try PT and PM....first b4 considering surgery to see how the chiari is affecting ur DH.,.....be open to what they r saying and learn all u can.