Hi, it is possible to take medication and still have seizures , I myself am an example of this, just because someone takes medication for epilepsy it does not necessarily mean that the medication will totally control it.
There are approximately 20- 25% who dont benifit from full control with medication. Your dad may fall into this category or may need to see a nuerologist and get his medication reviewed ,it is possible that maybe the dilantin does not suit him any more or may need his dosages adjusting.
He may get better control if he took half his dilantin in the morning and the other half in the evening ,this would result in a more even level of dilantin in his system rather than having a large dose in the evening .
Good luck