My 2yo cats right eye is watering at a steady pace. Conjunctive? Or maybe you have an idea...?

I have a 2 yo male maine coon / mix breed. He is typically an indoor cat, but on occasion seems to find the outdoors. He has not been fixed and has not had his preliminary shots. Today after he came back from a 3 day vacation, I noticed that his left eye is watering profusly. Not heavily but steadily. What is the reason for the watering? Eye looks unswollen and normal.