He probably has something in the eye that is irritating it. I would see the vet ASAP before the problem gets worse.
Is the white of the eye irritated at all? Is he holding it closed, winking, or behaving any differently?
Max went through some eye problems. He had corneal ulcers in both eyes. I thought he was okay until one of his eyes started watering a lot. It turned out that the previous irritation and meds had caused a tiny hair to grow underneath his lid. The hair was a new irritation and scratching the cornea. He had to have it removed with laser surgery (which isn't as bad as it sounds).
I am now an expert of being able to find tiny stray hairs in the eye and corneal ulcers.
Best wishes for your little one.