Please check with your son to make sure that he has completed all of the steps in this Apple Support document:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
The old phone may still be locked to the carrier that your son was using, so the carrier will need to unlock the phone before it can be set up again as a different device.
Then, you can decide on which carrier that you might want to use with the phone. If the carrier has a local store, it will help speed the process if you take the phone in to them.