The usual seizure lasts three to five minutes and is harmless to the child. Febrile seizures are essentially impossible to prevent. They generally occur on a rapid upswing in the temperature, often before the parents are aware the child even has a fever.
Most children only have one or at most two such seizures in childhood.
They do not cause epilepsy later in life. Febrile seizures are limited to the young, probably under three years old. A seizure with a concomitant fever in an older child is a different animal and may indeed be related to epilepsy. Don't worry, he'll perk up soon and be his old self. After exam and any lab tests, if it is clear that only a febrile seizure was involved, the doctor will probably let your child go home on whatever medicine is appropriate for the infection that caused the fever and will recommend some pretty aggressive fever control.