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Another reason is that there is a problem with the mouth. It might be an abscess or tooth problem.
Some dogs are prone to seizures and will drool excessively when this is going on.
There could be a female in heat somewhere who has gotten him excited.
If he is eating and drinking well, you can wait and see the vet later. If he's not, you will need to make an appointment tomorrow.
Let me know if you still have questions.