Also, I probably should put in an author's note about the cause of Zach's amnesia in the back of the book. Years ago, when my sister was about 17, as she was driving a dead rabbit crashed through her car's windshield and shattered it. (We lived in rural Washington.) My parents decided that a high flying eagle must have been carrying the rabbit and for some reason dropped it and sent it crashing with enough terrific force to shatter the windshield. That's why I mention the dead rabbit lying beside him, but most readers won't put that together.
There's always the other, more probably, explanation. When you've had a serious head injury, it's not unusual to have sudden blackouts years and even decades later. Because of his football injuries, Zach could have easily blacked out, fallen, and struck his head.
Of course, neither Zach or Charlie would know either of these things.