My neuro encouraged me to keep it in place, saying that no harm will come since the device was turned off. I had it explanted anyway. It was a constant reminder of my near death experience with the device. I hope your son's doctor explained that the electrodes are seldom removed due to the dangers involved and scar tissue issues. Scar tissue from the electrodes can also cause problems later in life. I'm dealing with that now.
Chances are you and your son were not informed of this prior to surgery. That seems to be Cyberonics' modus operandi.
You should check out the Programming Malfunction Sectionhttp://www.vnsmessageboard.com/index.php/topic,3975.0.html
If the VNS can change settings on it's own then it's conceivable the VNS can reactivate on it's own. Take my advice and have it removed.
WISHING YOU AND YOUR SON THE BEST