As many dodgy ones as you can find.
Also, try posting the email address on a Geocities page with a bunch of search terms (so that hopefully, any spammers who harvest email addresses from the web will pick his up). Post some messages on newsgroups 'using' his email address too - I reckon spammers get addresses that way too.
Another thing you could do is find some web message boards that don't seem to log IPs rigorously, post some contentious messages 'using' his email address as the public address (e.g. white supremacy on civil rights boards, civil rights on white supremacy boards)...