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There are lots of reasons that a feline may drool. The most common is having something in their mouth that either tastes bad or is causing pain. A painful mouth may be caused by ulcerations from licking something frozen, from licking something too hot, by consuming something that shouldn't have been eaten (ie potpourri, sticker-like plant material, bugs, etc) or by dental disease. Drooling can also be caused by nausea or fear.
As far as diseases go, the most notable one is rabies. This is caused by the reduced capacity to swallow, thus leading to drooling (rabies does NOT have to produce the stereotypical "foaming at the mouth"). Some pets with kidney and liver disease will also drool profusely.
If you do not know this cat, I would advise keeping a good distance and calling animal control in your area to provide him with assistance. Even if it does turn out to be something as simple as something stuck in his mouth, it's best to exercise caution in this situation.
Learn more by visiting: http://www.cathealth.com/drooling.htm
The answer was provided slightly over an hour after being asked, I apologize that it was not sooner. I was not online.
There was no detail to the amount of time that this had been going on. You gave the age of the cat, but no detail to how long this had been an occurance.
If the cat has been drooling for over a year, it's very unlikely to be rabies. It's more probable that the cat has dental disease or something more long-standing such as stomatitis (which could cause ulcerations, which could cause drooling). If this were a stray cat at my home, I would have him examined by a veterinarian to determine why the drooling was occuring. If dental diseases is diagnosed, a dental cleaning (and extractions, if needed) would be an appropriate treatment to relieve the problem in the mouth.
Here's a link to stomatitis: http://www.peteducation.com/article.cfm?c=1+1371&aid=368
And one to dental disease: http://www.peteducation.com/article.cfm?c=1+1371&aid=378
Truly, without having an exam performed, there is no way to know what is causing the drooling or what can be done about it.