Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with a doctor.
Without the ability to examine and obtain a history, I can not tell you what the exact cause of the symptoms is. However I will try to provide you with some useful information.
Seizures of any type can certainly occur during sleep. There are some seizure disorders that are more likely to occur during sleep, others that do not necessarily correlate with time of day, and others that are more likely to occur in the morning on awakening, but there is no definitive rule, and any seizure can occur at any time. Myoclonic seizures, such as occurs in juvenile myoclonic epilepsy, are of the seizures that commonly occur in the morning, but can certainly occur at night. If there are concerns that seizures are occurring at night, padding around the bed to prevent injury during the night is important. If there are concern for frequent seizures that have not been witnessed, videotaping the patient during sleep is one way of capturing such events; evaluation in an epilepsy monitoring unit (where the patient is recorded over night sometimes for several days) may be indicated depending on the exact history, diagnosis, and seizure frequency.
Discussion of your concerns with your son's epileptologist is recommended.
Thank you for this opportunity to answer your questions, I hope you find the information I have provided useful, good luck.