It's probably a hay poke. From what I can see of the picture, there's no wound to the eyeball. If that's correct, I'd just watch very carefully, but get him to a vet if you see any signs of infection -- pus, swelling, heat, etc. Redness is also an indicator of infection, but also of irritation, and if he's just been poked in the eye, it's very apt to be irritated.