aa18

He can travel with his passport and a  copy of his  I-485 receipt. Rarely, CBP checks for I-485 receipts of aliens  who have neither a valid  non immigrant status nor a valid immigrant status. And these checks can happen to pax traveling domestically.  Keep in mind that CBP has this authority within 200 miles of the border and the border includes coast line.  
 
See 
 
 
and
 
https://www.reddit.com/r/immigration/comments/pvrtx5/asked_to_prove_immigration_status_to_fly/
 

Wasn’t a problem for us when we flew from New York to Florida in July, I was still waiting for approval for the AOS, so as long as it’s pending, it will be fine 🙂. Take a copy of the NOA1 if you’re worried, for peace of mind but you shouldn’t need it!

What I meant by "out of status":
 
1. If he overstayed his B1/B2 visa or ESTA
2. If he dropped out of college while on F1 student visa
3. If he left his employer and doesn't have a job while on H1B
 
You get the gist, I hope.
 
If he is out of status, I would ask a more knowledgable person. If he is in status (maintaining his visa etc) then should not be a problem at all.
 
Just have his letter in case if he need to explain his status to officials (which he shouldn't need to do in most cases I would believe).
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Sorry, I misread this is K1/K3.
Ignore me. Should be fine.

You only need your ID or DL for domestic flight..just book your flight/ present that at airport 

*** Moved from AOS from K1 Visas forum to Working & Traveling During US Immigration, where similar topics are discussed ***