What do I do with my 21yr old epileptic son who absolutely refuses to take his antiseizure prescription?

Ask him why. Ask your son why he refuses to take his medication. He may have legitimate reasons, drowsiness, difficulty concentrating, double vision, ambulatory difficulties. He may have to take his medication in front of his peers (not cool), and may need to change to a different medication or different schedule. Having a seizure in front of your peers is not good, adjust his medication to fit his schedule.
Thats tough. This is a tough one. When people reach adulthood, they think they know their bodies better than anyone, and they don't always want to do what is right for themselves. We cannot force feed his medicines to him. But he should not be driving if he has seizures and is not taking his meds. I would do what you can to stop him from driving.