There’s a few roadblocks standing in the way of your plan. First off, they will start a new policy with the iPhone 3G where it will be very hard to buy them as gifts. You will be required to activate the phone before leaving the store. So your son won’t simply be able to walk in and buy one and walk out- he’ll actually have to give account info right then and there. Secondly, in order to port your phone number, they will require your presence in order to get your permission. Otherwise anybody could just steal your number and cancel out your Verizon account, you know?
Maybe there’s some loophole that will allow you to do what you want, but AT&T and Apple are intentionally requiring in-store activation to curb the unlockers out there, and people like you are going to fall victim to the new policy.