Since you have indicated that this only involves just one of his eyes, this irritation has most likely arisen from something that has gotten under the eyelids, blown into the area around the eyeball, or his eye has been bumped or abraided by something. Any of those can cause a reddening of the white part of the eyball. As long as he is not squinting with that eye, he probably doesn't have a foreign body lodged anywhere under a lid. If you can do it, a gentle flushing of the eye with some over-the-counter eyewash several times a day should ease the irritation. However, if it doesn't improve within another day or two OR if he starts to squint with that eye, he needs to be examined by your veterinarian right away.
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