My daughter has had several febrile seizures. She had her first one when she was about 13-14 months old and then has had one more since then. We found out after the first one that there is a family history of febrile seizures on my husband's side.
We also went to the ER after the first one, not knowing what it was. Once it was determined that it was most likely a febrile seizure, there wasn't much else to do except treat the fever and keep a close eye on her temp. When she had her second seizure, it was terrifying, but I knew that it was most likely another febrile seizure. We didn't go to the ER that time, but did immediately call our pediatrician and went in for an office visit.
Something our pediatrician told us about febrile seizures is that first of all, only a small percentage of children react to sudden fevers by having seizures. Out of those children, a small percentage will have more than one febrile seizure. The good news is that most children outgrow them by the age of 5.
I'm sorry that you're going through this with your son. Watching my daughter have a seizure out of nowhere has been one of my scariest parenting moments. I am slightly less paranoid about her having more seizures as she gets older, but I'll always be a little extra-cautious every time she starts to spike a fever.