well I don't always agree with Vets or Drs either for that matter, I prefer more holistic treatments whenever possible....
I really had never heard of that one before, and unfortunately it didn't give the results it was intended for.
I know its hard to tell who's poop is who's when you have multiple cats however when you are scooping out the box the rubber band should be quite visible tangled up in the poo, so keep watching for the next few days.
If he doesn't pass it, theres always the possibility of it becoming tangled up within the bowel so you would need to again consult a Vet with how to move forward.
You may need to put on gloves and dissect the stool to see if its broken up into small pieces therefore not as visible as a whole band would be....
you can also post this question to our Medhelp Vet and see what he says. just click on this link. good luck. keep us posted okay.
http://www.medhelp.org/posts/new_with_new_subject?forum_id=112