I’m sorry to hear that your son is dealing with epilepsy! Unfortunately, if someone has a seizure while driving, there isn’t much that you can do as a passenger. You can try to grab the steering wheel to keep the car on the road, but you won’t be able to do much to help your son until the car comes to a stop.
It’s often recommended that people prone to seizures refrain from driving whenever possible. However, since epilepsy usually has very specific triggers, it may be safe for your son to drive as long as he avoids these triggers. Driving at night, for example, would be more dangerous for your son than driving during the day.