Please report your specialist if this was in their report are you certain that is what they said. This is irresponsible and fear mongering to say the least. There is no way your doctor could have determined a fever caused your child's autism specifically. A simple medical journal search would have confirmed the "causes of autism" ranging variations of hereditary, non-herediary, external environmental factors and so forth. It's time for a new doctor. I'd also recommend you attend some therapy, this is definitely an irrational fear you have developed.

Sorry if this sounds harsh I'm just annoyed with doctors that assume people are incapable of understanding a simple response of them no knowing why your child has autism.