I would get him to a emergency vet right away! Drooling isn't a good sign if I remember somene from my other forum said there bunny was drooling etc and went into a seazure. Wich later occured in it dying.

When bunnies go from being totally normal to being sick so drasticly it usually is a indication of something VERY wrong. They hide the fact that they are sick very well so any slightest sign is a bad one and they should be to the vet immediatly.

I'm not saying this will happen or trying to scare you or whatever but when I read something like this on other boards the bunnies usually end up dying that night or the next morning. Once a sign is shown they don't usually live for much longer unless they get to a vet right away.