Does he appear to see out of that eye? Sometimes there can be eye abnormalities in the iris which can affect the shape of the pupil. In ocular Mareks disease, the pupil can be irregular or pinpoint. There are many other causes of eye disorders, such as infection, injuries, cataracts, and others.
He seems to use his other eye to really focus on things. There may have been trauma to this eye, but otherwise they're isolated from other fowl. I don't think this pupil is normal. It also is not reactive to light. How would I know if it was Marek's?
REally the only way to diagnose Mareks is from a blood test (PCR test done by Texas A&M, NC State U. or other center) or from a necropsy done by your state vet. If you have ever noticed paralysis or unexplained deaths in your flock, there could have been Mareks, but just by having an odd eye like that, I would bet that it was just an injury, birth defect, or infection that caused that. Here are some pictures of Mareks eyes (note that the one in the center is normal