First off, great job hanging out with your son. Way to go Dad!

The setup I'm getting ready to use is like many have suggested, but there's another consideration. My battery back-up can recharge the iPhone 8+ a couple of times. I stash it in the under-seat bag with the cable zip-locked up to the phone holder on the bars. The iPhone itself is set to "low power mode" with the screen on minimum brightness and the auto screen
lock set to 1 minute. The fully charged iPhone has lasted over 2 1/2 hours on its own, so I expect to be able to get thru my first century ride with no power issues. During our rides, while i'm drafting as much as possible, I'm only interested in "time" and "averages" and I only check on those every so often. The combination of a battery and iPhone settings seems to do the trick.