This page may be useful.
http://www.doctoryourself.com/whoop.html
I've heard that Homeopathy and Chiropractic help alot, but don't know anyone whos used it. Have you enquired at the yahoo vaccinations group AVN aswell?
I hope your son is better soon!!! I had Whooping Cough pretty badly a few years ago and it was awful, but my Mum and Son only had it mildly.