might be illness. Pasturella can sometimes exhibit like this. A persistent runny eye is something to cull for.
Might be something in his eye. I would very carefully examine it. IF you can see something, treat it with a lukewarm tea bag to the eye... the astringents in it help the eye to heal.
Things to look for... a bit of dust or hay particle, a scratch etc.
Might be indicative of a tooth issue. sometimes the roots of teeth can cause a problem with the eye. This in my book is also a culling issue.