Well, given that he seems to be doing fine now, the cause of this may remain a mystery.
Often times, the one sided pupil dilation (called "anisocoria") can be a sign of a life threatening condition -- but those people are usually unconscious with a dilated pupil.
Other times the pupil can be dilated from drops of eye medications, like those given at the optometrist's office.
Conditions like "Horner's syndrome" can also cause dilated pupils, as well as glaucoma, uveitis, or injury to the eye.
Because irritation of the blood vessel (carotid artery) on the same side as the eye, the nerve to the eye (oculomotor nerve), the eye itself, or inflammation of the iris itself can cause this, a complete evaluation is usually done when someone has a dilated pupil. Of course, now that it is gone and he is feeling fine, it may be difficult for doctors to find a cause. Of course, should it happen again, seeing a doctor would be helpful to find the cause.