Your child had a fit, convulsion or seizure during sleep. The seizure occurs due to sudden excessive, electrical discharges from the brain cells called neurons. As the electrical discharges reach a threshold and spread to other areas of the brain, a seizure results.
The reasons for a seizure may be genetic, infection, head injury, residual damage from birth injuries or difficult deliveries, structural abnormalities since birth, low calcium, sugar etc; tumours etc. In 30 -40% the exact cause cannot be defined.
I would suggest that a good quality EEG be done that incorporates both wakefulness and sleep. Depending on the results of EEG, blood tests, CT/MRI of the brain MAY be required.
If tests are normal and there is no family history of fits, one may not start medications, as up to 30% of patients may not have a fit again. If he has a fit again, medications to prevent further attacks will be started. These will continue for a 2 year fit free period, at the least.