Yeah, these days buying a phone outright if you can is fine.
The build quality and longevity is pretty uniform now and they will last years, battery life may be an issue at some point, but not for a while. Upgrading every year is pretty pointless, my first iPhone is still going as my second phone, my 6S should be around until about, 2018/19?
Buying upfront means you can shop for any SIM deal you like, and if you find a good short term PAYG tariff, you can switch sooner than on bill-pay.
I know in the USA you have two main networks, but there are bound to be a number of deals available for different SIM plans.
Let him use Android 5.1 and iOS 9.2 now, and see which one is easier to use, because the GUI is what you'll be looking at and using for the life of the phone.
After that, look at SIM plans, battery life, memory storage and build quality, pick the best option out of that.