Have you ever watched a teen using a smartphone?
They ALWAYS use both hands, no matter what size the phone.
Both my girls have the XR which is big, and they show no signs of fatigue, RSI or whatever. On the contrary: I can’t give them a big enough screen
My suggestion would be: give him the 8+, and use the 6 yourself until the new iPhones this fall come out. Then you have options a-plenty to choose from.