With the age of the kitten, I am inclined to agree that something else other than rabies is likely to be the cause of his illness. Rabies can have a long incubation period, and even with a shorter one, the timeline would have the kitten being bitten even before he was weaned (like at 5-6wks old).
One another note, if he has been treated for fleas - these symptoms are also similar to a flea medication overdose. Drooling, very ill, sometimes diarrhea...