If your son ships the pistol to Colt, it will need to go UPS or FedEx Next day- about $70. However, you can find a DEALER in your area that can MAIL it by US Mail- it is legal for a DEALER or MANUFACTURER to mail handguns- but not us unwashed masses types. Even after paying a dealer for his time, and a $10 one price Priority Mail box, it will be cheaper. My advice- send it back. PS- go to gunbroker.com, use the "find a ffl" feature, based on his zip code, listing of dealers that have registered as doing firearm transfers. colt can return the sidearm directly to him when finished.
What we have here is... failure- to communicate.