This is known as the photic sneeze reflex.

It is inhereted in an autosomal dominant fashion. As there is very little room inside the average persons head lots of nerves come into close proximity to each other. In the case of photic sneeze reflex, this is most likely to be caused by the nerve responsable for sneezing being too close to the optical nerve. Any time the optical nerve tries to pass a strong signal (such as an intense light being experenced) the signal may jump to the sneezing nerve.

You could try asking if one of his parents has this same reflex. It is quite likley that is where he got it from.