12 hours ago, RedCardinal said:

Thai son with my family name in his Thai passport.  Most recent departure 3 weeks ago.  He's 10, and IO asked him a bunch of questions in Thai about where is his mum before allowing him to leave.

 

I've always been asked for letter from his mum saying he can leave.  One time they called her phone to speak to her from the immigration desk, despite me having all the docs from family court showing I had joint custody (not married) and letter from mother.  That said, I've never had anything stamped by Amphur.  I've always taken his birth cert, copy of Family Court order, letter from mother with copy of her ID.  Has been sufficient every time.  Him having my family name might have eased this.  I've never been asked once for anything when departing my EU home country with my son alone..

 

So for anyone saying OPs doubts are unfounded: YMMV.

I'm expecting some questioning like that, but I'll be as prepared as you. 

Just updating: I made it past the passport control. The booth officer immediately sent me to the table in the back and a very friendly lady asked to see copies of my son’s birth certificate and my wife’s ID (I had both with wife’s signature) then I called the wife on FaceTime (she was still at the airport) and after a couple basic questions about the trip we made it through.