You need to remove him from the current organization, if you don't have access you should ask someone who does to remove it for you.
It's not worth asking Meraki support, they don't have authorization to remove it for you, the most you'll get is to know the name of the organization to which the device is linked.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.