Diagnosis of photosensitive epilepsy

Hi, my son (6) is currently undergoing testing for suspected epilepsy. (He has a chromosome disorder with a 1 in 3 chance of developing epilepsy) He has excessive nighttime twitching, but has also had six episodes which seem to have been triggered by light through trees/hedges where he says his vision goes all one colour and one episode with flashing lights where he went blank and started kicking his legs. He had an eeg with strobe testing, but this did not trigger anything. I was wondering if anyone knows if it is possible to have photosensitive epilepsy that is not triggered by that testing. I will speak to my GP about it, but next appointment is not for a couple of weeks.