I’m not saying you’re making the wrong move, but if you’re willing to go with a carrier like ATT, you can get $1000 trade-in value for that iPhone 13 Pro towards a new iPhone 16 Pro. You can even just buy an unlocked iPhone 12 off of eBay (for about $250) and get the same $1000 trade-in credit for you son. There are some caveats. For example, the credits are paid out evenly over 24 months, but if you plan to keep it for 2 years, you basically get a $250 iPhone 16 Pro.
Again, it might not be the right decision for you, but I thought you might like to be aware of the option.
I've tended to buy iPhones that are 2 or 3 generations old from eBay and Swappa for my family and use Mint or Tello for cheap cellular service. Our costs might be $350 for a phone and $100 - $150 per year for service.
We do get them a nice new phone when they graduate high school.
Again, it might not be the right decision for you, but I thought you might like to be aware of the option.