Our son has had 2 febrile seizures in the past should I be worried that this will occur with every high temp?

Not really. Febrile seizures occur in about 2% of children and do not have a risk of causing harm unless very prolonged; it's not so much how high his temperature goes, but rather how fast it goes up. Having a history of febrile seizures only minimally increases the risk of later being diagnosed with epilepsy.