You can do it like this:
1. Take new phone out of the box and do NOT turn it on.
2. Remove the new sim and set it aside.
3. Take the sim from your 5s and put it in the new iPhone.
4. Turn it on and set it up however you like.
5. Erase your 5s and send it to your son.
6. He inserts his 5 sim into his "new" 5s and he sets up it up.
At no point do you have to call AT&T. What do you do with the sim that was originally inside the 6? Nothing!
Yes that sim is tied to his number but it's never going to be activated so his current sim will stay working. Since att/apple is mailing you a phone, the sim inside has to stay inactive so you can continue to use your current phone. It would have to stay inactive indefinitely because how they don't know how long it takes to get your phone. What if you are on vacation, what if it gets lost in the mail, etc?
I used this exact procedure going from a 5 to 5s and will do it again today with my 6. The upgrade I stole that time from our large corp account still has his/her phone working after a year and the original sim from the 5s is still sitting in a drawer somewhere. So at the end of today, the sim in my 6 will have gone from 5 to 5s to 6 without ever calling AT&T.
Bam!
thank you. Exactly what I am going to do!
Appreciate the detail in your response, seems to be a ton of confusion. Even on ATT.net they say you have to go into the store or call, why I have no clue.