Probably he did a “Find My Friends” thing on Facebook where it scanned his entire email address book. You may not have been on Facebook at that time, but your name and email address was in his address book so it was noted down deep in the Facebook meta-conscience.
Some time later you come along, either using that email address or with some link to it, and Facebook spots the connection.
This will be it. Linkdin and other social networks do this too. Its not a one way relationship, you maybe didnt have any ties to him or let FB access your email, but he maybe did.
I laughed recently when FB suggested I tag a face in a photo I’d uploaded. The face was in a painting in the background of the photo.
There is no way he had my email address though.
Mutual contacts would also trigger this sort of thing. FB basically builds a graph of connections and then extracts and extrapolates from it. You never notice the ones it gets wrong, but the ones that are right appear to be spooky.