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I am sorry to hear about this concern.
Drooling can be caused by several issues including recent toxin exposure, periodontal disease (bad dental problems), a hypersensitivity reaction (like an insect bite or sting), nausea from something foreign being ingestd, to name a few.
So, drooling can indicate a very serious concern. At only 12 weeks of age, I would really consider a ER vet visit to ensure he is okay. I would not recommend anything over the counter for a dog of this age, and I would be cautious taking a wait and see approach. Not much room for error at only 12 weeks of age.
Hope that information helps