How would you prepare a six year old to fly by himself?

Please no reprimands on how he is too young. I don't want him to go, but my in-laws already purchased a ticket and my husband had told them it was fine before I knew about it. He sees no issue, he and his siblings all flew by themselves from 5 years old and on, to boarding schools and camps. My son has never flown before, now his two older brothers (10 and 11) have both flown many times by themselves. He is a very well behaved child, but how do I prep him for the loud sounds and the large bumps and the feeling of being shoved back into your chair. Any help without judgment is greatly appreciated. He is flying Phoenix to Portland, Oregon with no stops or layovers.