fainting seizure ? with flu shot

HI my 12 y old son recently had a flu shot and immediatly passed out and had tomic clonic like jerks with eyes open but not AxO for about 30 seconds with no postictal period. When he came around he was AxO 4x4 but diaphertic for about 30min. He had a similar response to a measles vacc at age 2 but has had several vaccinations and shots since with no problem I am a paramedic and have no explanation nor can dr's offer one other than a vagal response but other than the absence of a postictal stage it presents as a typical siezure and doesnt resemble any vagal response ive every seen. Any thoughts on what this might be or something with the way these shots are administered or mixed with ? thanks in advance.