Sabrina - which email domain are you using?

Maybe, as a "clean slate" step, you can get a new email account and give out the new address from now on. From the old account, have auto-forwarding to the new account. Send a mass mailing to all your contacts from the old email client, excluding him and any mutual contacts, announcing the new address.

Is that possible?

Another idea is to have his emails returned to him (bounced back), but not every email client supports this function.