Assuming he is flying domestic, within the United States, no paperwork or documentation of age is needed. Southwest and TSA may ask him his name if he is able to say it, and then they will look to see if it matches what is on his ticket. He can earn Rapid Rewards points so it wouldn't hurt to get an account for him and add his RR# to the reservation. You can also request credit for the flight after it is completed if you don't add the RR# before the flight.
