Why don't you try (just a thought) tape recording him and playing it back one day out there to see if any others respond.
Or you can tape record one from the internet (an unfamiliar voice)....or just put your speakers from the computer near the window and play it loud. Your neighbours may object but hahaha it may work.
Check this out, you can listen to many bird calls here:
http://www.enature.com/fieldguide/sh...&recnum=BD0023