My Lady is an epileptic. She was diagnosed at four years old.
You need to take him to your vet and get a complete physical, including bloodwork. He should also get a bile acids test to test his liver function and a thyroid panel. Both liver shunts and a thyroid imbalance can cause seizures. It isn't an emergency situation unless he he starts having several seizures in a day so it can wait until Monday.
Yorkies are 36 times more likely to have a liver shunt than all other breeds combined so it is essential that he have a bile acids test done. Seizures are one of the symptoms.
Here is wonderful resource about canine epilepsy:
http://www.canine-epilepsy-guardian-angels.com/