@nin
I have never seen a Neurologist at or been in the Mayo clinic Comprehensive Epilepsy Center but was in one at Stanford University and one in San Francisco.
I believe your best chance for diagnosing the cause of your sons epilepsy would be at a Comprehensive Epilepsy Center where he would be under the care of an epileptologist. Normally Epilepsy Centers see patients with intractable Epilepsy. However, since his Epilepsy isn't active I wonder if they would accept him as a patient.
Benefits of an Epilepsy Center is they work as a team with various specialists and will decrease or stop his medication so he'll have a seizure which will be captured with video EEG to hopefully locate where the seizures originate. They have the latest imaging technology, surgical techniques and Neurostimulation devices.
Since your son's seizures are no longer active why is it important to find the cause?

Mayo Clinic contact information:
https://www.mayoclinic.org/about-mayo-clinic/contact
List of Comprehensive Epilepsy
Centers throughout the U.S.
https://www.naec-epilepsy.org/about-epilepsy-centers/find-an-epilepsy-center/
Take care,
Jake