I'm just shy of 64, have the flattest feet of anyone ever. Full footprint out of the shower, when I walk on the beach, where my arch should be actually supports most of my weight.
Flat feet equals longer feet than arched, so shoes need to be narrower.
Problems....when he hits 60, his ankles and feet will bother him somewhat. When he hits mid 60's, he will need orthodics for comfort, but not necessarily for sports.
I played 3 years of high school football, 2 varsity, one JV, and was scouted by both local colleges when I was a junior. Also played 3 years of varsity basketball for high school, always starting at least.
Was a long jumper in junior high, baseball and basketball.
No, it didn't bother me until I turned 60.
I knew tons of players with high arches who had more problems with their feet. Most of my teamates in basketball, mostly black, had very flat feet, as did the DB's and CB's in varsity football.
One noticable factor.....seems flat feet doesn't hurt sprint work, but kills you if you chosen sport is one of endurance, like marathons or 10k's.
And swimming sucks, as people with flat feet can't point their toes to get a kick that pushes forwards, and most cannot ever possibly past a DUI test standing on one foot.