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If your son had a fall landing on his neck he may get pain due to a possible internal injury
and to get relief it may be that he is stretching his neck because keeping it in a particular position may be helping him so to avoid the pain he is doing it.Normally it happens that when any person tries to take a particular position to avoid pain or excess pain he may land up doing it involuntarily as a reflex to the pain.Please do not worry and keep giving him pain killer when required and by the time 7th July appointment comes you can get a X-ray of the cervical spine
(neck) done and you will get the report and also come to know whether there is any injury or fracture
which is bothering him.With the X-ray you may consult the doctor.