My biological son who holds a US passport will be traveling with his mom to the US for the first time this month. He has a US passport and a Thai passport. She will be traveling on a K1 visa. 

 

Do I use his US passport number when booking the airline ticket?

 

Will his mom just show his US passport to Thai immigration on departure to US? Or is there more to it?

 

Thanks

Use the US passport at airline checkin (make sure name on ticket and passport match, very important), exit Thailand as Thai. Enter USA as USA. Then reenter TH as Thai.

 

You would show the U.S. passport at airline check-in.   But if for some reason you booked the ticket with the Thai passport the airline will probably ask to see that passport.  You can show both if asked to satisfy the airlines the person can enter the country they are traveling to.   The airlines just wants to confirm the person is who they say they are and has the proper documentation (i.e., passport/visa) to enter the destination country.

 

But after than when going through immigration points, show the Thai passport at Thai immigration (exit and entry) and the U.S. passport at U.S. immigration (exit and entry).  My dual citizen (Thai and U.S.) wife has been doing this way for years in travels between Thailand and the U.S.