Hi and welcome to the Chiari forum.
I am so sorry u r so distressed over ur little one...and I understand ur worry.
Until more tests r done, and u know more try to stay calm.
Yes a brain MRI is needed as is a CINE MRI...this is to check for a CSF obstruction and overcrowding...u said he had a spine MRI do u know was it a full spine or just the cervical spine? If just the cervical u should also request a thoracic and lumbar spine MRI to ruleout a syrinx and tethered cord.
And while doing tests, make sure he is checked for sleep apnea, ehlers-danlos,and disk issues, and ICP.
The biggest obstacle u will have is dealing with the drs until u find a true chiari specialist.
We do have a list of Drs that u will need to research the drs as the list is not a referral.
http://www.medhelp.org/health_pages/Neurological-Disorders/DOCTORS-LIST-FOR-CHIARI-SYRINGOMYELIA/show/1062?cid=186
"selma"