At this point, it is not possible to confirm whether someone has an autism spectrum diagnosis with an EEG. I would encourage you to speak with your child's pediatrician to clarify the purpose of the EEG and to ask for a referral to a developmental psychologist or another professional who can do an evaluation of your child's skills and behavioral presentation. Only a trained professional who has experience evaluating individuals with autism can provide a valid diagnosis.
My EEG was classified as normal without it having any implications for my autism diagnosis.
It is for sure possible that autistics may have a different EEG pattern, but it does not exist enough science yet to support it and so such use and measure of EEG does not exist at a serious clinical level but may be used serious in research. For that reason only the standard and valid use of EEG was taken into account when I was tested and that standard and valid test did not have any implication for my autism diagnosis.