Pertinent questions that need to be answered are: How persistent is the "flash"? (That is, is it something that happened once and disappeared? Is it something that happens occasionally or frequently, but is only a brief flash of light that comes and goes away? Or is it a persistent light which reappears whenever he pays attention?) And, if he moves his eye when it happens, does it lag behind the eye movement and then swim around to be in the center of his field of vision, or is simply always immediately in the center of the field of vision no matter where he looks? You only need to sorry about it if it is a chronic or frequently recurring thing. Something which lags behind focus by a little bit is likely to be something on or near the surface of the eye; something which is always immediately in the center of the field of vision could mean it involves the macula, the central part of the retina which gives us the high-detail, high sensitivity focus in the very center of our field of vision. There are a number of things which could these, ranging from a small patch of congenitally abnormal cells to exposure to abnormal growth of blood vessels under the retina, but a diagnosis simply can't be made over the internet. Your best bet is to find a good ophthalmologist or even optician who can take photos of the inside of the eye to look for signs of any abnormalities.

If it's only when he closes his eye its harmless. I have the same thing and it doesn't bother me at all. Hwever if it's when his eyes are open I suggest taking him to an optometrist or doctor.