It is so scary when your child has a seizure yet it is not uncommon. There are many different types of seizures but I am guessing your son had a tonic/clonic or grand mal seizure. Like the ones you see on t.v. Seizures times are different, but if they last less than 5 minutes you don't have to call an ambulance. You can just call your doctor. Afterwards it is very normal to be disoriented for awhile and very sleepy, but the seizure is over when the shaking stops.
The onset of fever can cause a febrile seizure. If your son is sick my guess is that he had a fever, but I would have him evaluated by a child neurologist all the same. Pediatricians are wonderful but they are not experts when it comes to seizures. Find yourself a specialist to be sure your son is being diagnosed appropriately. Medication may or may not be needed and a neurologist will know about al of them, and should give you some choices and explain side affects, etc... If this was a febrile (or related to fever) seizure then it is unlikely your son will need medication AND it is likely he will outgrow the seizures.
If you don't know it already then you should learn seizure first aid to protect your child and be sure teachers and babysitters are also aware. Here is a link: http://www.epilepsy.com/epilepsy/firstaid
Let me know if I can help. My son has seizures, and though I'm no expert, I have experience and understand how hard it can be on Mom.