I hope he was given instruction (from the ER) about post-concussive syndrome. If his memory is just returning now, he really needs to take it easy. Maybe I know too much about TBI from neuroscience, but I have a close friend whose teenaged daughter took a hit playing soccer and after almost a year, she is still struggling with academics/memory, etc. I know this is an unusual case and she has been cleared to run, etc., it doesn't take much. I am sure he'll be fine and maybe he will take your instruction better now.
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