Yes, there is a risk of head injury in football, but there is significant risk in many contact high school sports; hockey, rugby, lacrosse, even soccer... I played in the 80's when the equipment was suspect, practices were always full speed contact and you were told to get back in there after "getting your bell rung". Equipment, technique and practices have evolved, for the better, over the last 25 years but in a contact sport there will be risk. My son has played since he was 10 and decided on his own to stop at 18. Going into university he said he just didn't want to play summer ball and risk (any) injury. All you can do is educate your son and let him make his decision. Sounds like this is your approach.