First off, do NOT choose Verizon.
If you must, choose an HTC, Palm, or Blackberry. Don't settle for the cheap Samsung, LG, or any of those because your son will absolutely hate it. The touch screens on most of the phones that Verizon offers are just terrible (except HTC, Palm, or Blackberry.)
Now, just go to the Verizon store and they will help you set up his plan and contract.