Hi XXXXXXXXXXXXX~
If the discharge from his eyes is new, then you should have him looked at by your regular vet, as this can be signs of both an ocular (eye) infection as well as trauma to the eye itself (such as getting it scratched somehow).
If he's always had runny eyes, then it could just be breed disposition, meaning that some small breed dogs do have mattery eyes, and Shih Tzus seem to be at the top of the list.
I hope this helps.
Certified Veterinary Technician
Associates Degree in Veterinary Technology. State and Nationally certified.