Causes of excessive tearing can be linked to genetics, and also to health, diet, and fleas. (Fleas nestled around the eyes often and they cause excessive tearing.
My own observation has lead me to observe a strong link to our dog's diet. In this case a fresh, raw meat/ and bone diet. The excessive tearing has diminished in numerous of my Papillons, since I switched from kibble to real food 3 years ago.
Also maturity of the skull will bring changes to the lesser tearing in some of the dogs.
Home treatment:
Keep your dog free of fleas.
Grooming: use a moist cotton swab daily to loosen up the accumulated matter around his eye.
Visit the ophthalmologist to find the most suitable alternatives for your pet.