Tracy, this is called anisocoria (unequal pupil size) and it has quite a few possible etiologies. The two most common causes are the leukemia (FeLV) virus and the spastic pupil syndrome - a condition unique to cats in which owners report anisocoria, which may sometimes be transient and independent of ambient light levels. Clinically, cats with this syndrome appear to be healthy, are visual, and have no ocular abnormalities beside unusual behavior of the pupils. Unfortunately, many affected cats test positive for FeLV at the initial presentation but not all which confounds the diagnosis.
Anisocoria also results from primary iridal (iris) disease - an example of an inflammatory process causing miosis (an abnormally small pupil); and primary neurologic disorders involving cranial nerve III and Horner's syndrome. Retinal disease - hemorrhage and/or detachment needs to be considered as well. If the retina isn’t perceiving light or telling the brain that light has been perceived, the pupil will become mydriatic – larger than usual - as you've reported. Retinal disease is relatively common in hypertensive cats and so blood pressure assessment is important in these cats. A vascular accident (stroke) is very unlikely at his age.
The initial database should involve testing for FeLV and a complete ophthalmic examination including menace response, dazzle, palpebral, pupillary light, and vestibulo-ocular reflexes. Fluoroscein staining of the cornea should be performed looking for corneal trauma and intraocular pressure should be measured (the elevated pressure of glaucoma will enlarge the pupil (mydriasis) and the lowered pressures of uveitis will make the pupil miotic).
Much of the above is beyond the capability of many generalist vets so your vet may recommend referral to a board certified veterinary ophthalmologist. Please see here: www.acvo.org
Please respond with further questions or concerns if you wish.