Not knowing a seizure happened is very normal. You should probably get him an EEG that would give you the most information. Just understand that we don't know a lot about the brain, so knowing why someone has a seizure isn't something that can be answered. Do not take what I'm saying in place of what a doctor will tell you. I want to stress that I'm not a doctor.
I was 21 when I had my first seizure. I have had 2 seizures since and I am now 31. I know, in my case, that my diet was very poor and I was under a lot of stress. I also wasn't sleeping well and was having muscle cramps. Do any of those things apply to your son?
Seizures can be caused by many, many things. People have varying degrees of epilepsy as well. Typically, things such as stress and a poor diet lower the threshold for having a seizure. That being said, people can have seizures for no reason apparent reason, or we have yet to scientifically explain why.
Chances are, even an EEG won't bring the neurologist any closer to understanding why this happened. If a person continues to have episodes, you can start to piece together similarities in some of the life circumstances surrounding the seizures. I hope everything goes well for your son. Thanks