I only read through about half the thread but...
Why is the focus at this point on the decision him or his wife made at all? That's already done and over with. This kid is old enough to be explained what the long term risks could be and make their own decision if the parents have already decided it's okay with them. My 12 year old wants to play football (ONLY BECAUSE HE WANTS TO BE THE JOCK...NOT BECAUSE HE'S EVER SHOWN ANY LEGITIMATE INTEREST IN THE SPORT, MIND YOU) and we have explained to him why we won't allow it.
I'm pretty sure if he can get that through his thick skull, your high schooler should be able to decide if they want to take the risk or not after being provided what we CURRENTLY know about CTE....which isn't nearly as much as we *should* know.