Oh, no! I don't have rabbits, but this sounds kinda serious. You should deffinatley call a vet right away. I have guinea pigs and the salvating sounds like slobbers. I don't know if that aplies to rabbits or not, but here's the info on slobbers anyway:

Slobbers
Signs/symptoms:Slobbering, drooling, weight loss, mouthe does not close all the way.
Cause:Overgrown molars possibly due to lack of chewing or hereditary malocclusion of molars.
Treatment:See vet. Molars may need to be filed as needed.

I'm sure this isn't what he has, but it is possible.
I hope he gets better soon;keep us updated.